Recently syzbot reported that unmounting proc when there is an ongoing
inotify watch on the root directory of proc could result in a use
after free when the watch is removed after the unmount of proc
when the watcher exits.

Commit 69879c01 ("proc: Remove the now unnecessary internal mount
of proc") made it easier to unmount proc and allowed syzbot to see the
problem, but looking at the code it has been around for a long time.

Looking at the code the fsnotify watch should have been removed by
fsnotify_sb_delete in generic_shutdown_super.  Unfortunately the inode
was allocated with new_inode_pseudo instead of new_inode so the inode
was not on the sb->s_inodes list.  Which prevented
fsnotify_unmount_inodes from finding the inode and removing the watch
as well as made it so the "VFS: Busy inodes after unmount" warning
could not find the inodes to warn about them.

Make all of the inodes in proc visible to generic_shutdown_super,
and fsnotify_sb_delete by using new_inode instead of new_inode_pseudo.
The only functional difference is that new_inode places the inodes
on the sb->s_inodes list.

I wrote a small test program and I can verify that without changes it
can trigger this issue, and by replacing new_inode_pseudo with
new_inode the issues goes away.

Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/000000000000d788c905a7dfa3f4@google.com
Reported-by: syzbot+7d2debdcdb3cb93c1e5e@syzkaller.appspotmail.com
Fixes: 0097875b ("proc: Implement /proc/thread-self to point at the directory of the current thread")
Fixes: 021ada7d ("procfs: switch /proc/self away from proc_dir_entry")
Fixes: 51f0885e ("vfs,proc: guarantee unique inodes in /proc")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

syzbot found that proc_fill_super() fails before filling up sb->s_fs_info,
deactivate_locked_super() will be called and sb->s_fs_info will be NULL.
The proc_kill_sb() does not expect fs_info to be NULL which is wrong.

Link: https://lore.kernel.org/lkml/0000000000002d7ca605a7b8b1c5@google.com
Reported-by: syzbot+4abac52934a48af5ff19@syzkaller.appspotmail.com
Fixes: fa10fed3 ("proc: allow to mount many instances of proc in one pid namespace")
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

syzbot found that

  touch /proc/testfile

causes NULL pointer dereference at tomoyo_get_local_path()
because inode of the dentry is NULL.

Before c59f415a, Tomoyo received pid_ns from proc's s_fs_info
directly. Since proc_pid_ns() can only work with inode, using it in
the tomoyo_get_local_path() was wrong.

To avoid creating more functions for getting proc_ns, change the
argument type of the proc_pid_ns() function. Then, Tomoyo can use
the existing super_block to get pid_ns.

Link: https://lkml.kernel.org/r/0000000000002f0c7505a5b0e04c@google.com
Link: https://lkml.kernel.org/r/20200518180738.2939611-1-gladkov.alexey@gmail.com
Reported-by: syzbot+c1af344512918c61362c@syzkaller.appspotmail.com
Fixes: c59f415a ("Use proc_pid_ns() to get pid_namespace from the proc superblock")
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

The current posix-cpu-timer code uses pids when holding persistent
references in timers.  However the lookups from clock_id_t still return
tasks that need to be converted into pids for use.

This results in usage being pid->task->pid and that can race with
release_task and de_thread.  This can lead to some not wrong but
surprising results.  Surprising enough that Oleg and I both thought
there were some bugs in the code for a while.

This set of changes modifies the code to just lookup, verify, and return
pids from the clockid_t lookups to remove those potentialy troublesome
races.

Eric W. Biederman (3):
      posix-cpu-timers: Extend rcu_read_lock removing task_struct references
      posix-cpu-timers: Replace cpu_timer_pid_type with clock_pid_type
      posix-cpu-timers: Replace __get_task_for_clock with pid_for_clock

 kernel/time/posix-cpu-timers.c | 102 ++++++++++++++++++-----------------------
 1 file changed, 45 insertions(+), 57 deletions(-)
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Starting from 2c470475 ("pids: Move the pgrp and session pid pointers
from task_struct to signal_struct") __task_pid_nr_ns() doesn't dereference
task->group_leader, we can remove the pid_alive() check.

pid_nr_ns() has to check pid != NULL anyway, pid_alive() just adds the
unnecessary confusion.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

With de_thread now calling exchange_tids has_group_leader_pid no longer
makes any sense and is equivalent to calling thread_group_leader.

As there are only 2 remaining users of has_group_leader_pid let's
update the code and get rid of has_group_leader_pid.

There is one extra patch to lookup_task that performs that unifies
to code paths that become identical when has_group_leader_pid went
away.

Eric W. Biederman (4):
      posix-cpu-timer: Tidy up group_leader logic in lookup_task
      posix-cpu-timer:  Unify the now redundant code in lookup_task
      exec: Remove BUG_ON(has_group_leader_pid)
      signal: Remove has_group_leader_pid

 fs/exec.c                      |  1 -
 include/linux/sched/signal.h   | 11 -----------
 kernel/time/posix-cpu-timers.c | 21 ++++++++-------------
 3 files changed, 8 insertions(+), 25 deletions(-)
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Now that the codes store references to pids instead of referendes to
tasks.  Looking up a task for a clock instead of looking up a struct
pid makes the code more difficult to verify it is correct than
necessary.

In posix_cpu_timers_create get_task_pid can race with release_task for
threads and return a NULL pid.  As put_pid and cpu_timer_task_rcu
handle NULL pids just fine the code works without problems but it is
an extra case to consider and keep in mind while verifying and
modifying the code.

There are races with de_thread to consider that only don't apply
because thread clocks are only allowed for threads in the same
thread_group.

So instead of leaving a burden for people making modification to the
code in the future return a rcu protected struct pid for the clock
instead.

The logic for __get_task_for_pid and lookup_task has been folded into
the new function pid_for_clock with the only change being the logic
has been modified from working on a task to working on a pid that
will be returned.

In posix_cpu_clock_get instead of calling pid_for_clock checking the
result and then calling pid_task to get the task.  The result of
pid_for_clock is fed directly into pid_task.  This is safe because
pid_task handles NULL pids.  As such an extra error check was
unnecessary.

Instead of hiding the flag that enables the special clock_gettime
handling, I have made the 3 callers just pass the flag in themselves.
That is less code and seems just as simple to work with as the
wrapper functions.

Historically the clock_gettime special case of allowing a process
clock to be found by the thread id did not even exist [33ab0fec]
but Thomas Gleixner reports that he has found code that uses that
functionality [55e8c8eb].

Link: https://lkml.kernel.org/r/87zhaxqkwa.fsf@nanos.tec.linutronix.de/
Ref: 33ab0fec ("posix-timers: Consolidate posix_cpu_clock_get()")
Ref: 55e8c8eb ("posix-cpu-timers: Store a reference to a pid not a task")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Taking a clock and returning a pid_type is a more general and
a superset of taking a timer and returning a pid_type.

Perform this generalization so that future changes may use
this code on clocks as well as timers.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Now that the code stores of pid references it is no longer necessary
or desirable to take a reference on task_struct in __get_task_for_clock.

Instead extend the scope of rcu_read_lock and remove the reference
counting on struct task_struct entirely.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

After the introduction of exchange_tids has_group_leader_pid is
equivalent to thread_group_leader.  After the last couple of cleanups
has_group_leader_pid has no more callers.

So remove the now unused and redundant has_group_leader_pid.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

With the introduction of exchange_tids thread_group_leader and
has_group_leader_pid have become equivalent.  Further at this point in the
code a thread group has exactly two threads, the previous thread_group_leader
that is waiting to be reaped and tsk.  So we know it is impossible for tsk to
be the thread_group_leader.

This is also the last user of has_group_leader_pid so removing this check
will allow has_group_leader_pid to be removed.

So remove the "BUG_ON(has_group_leader_pid)" that will never fire.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Now that both !thread paths through lookup_task call
thread_group_leader, unify them into the single test at the end of
lookup_task.

This unification just makes it clear what is happening in the gettime
special case of lookup_task.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Replace has_group_leader_pid with thread_group_leader.  Years ago Oleg
suggested changing thread_group_leader to has_group_leader_pid to handle
races.  Looking at the code then and now I don't see how it ever helped.
Especially as then the code really did need to be the
thread_group_leader.

Today it doesn't make a difference if thread_group_leader races with
de_thread as the task returned from lookup_task in the non-thread case is
just used to find values in task->signal.

Since the races with de_thread have never been handled revert
has_group_header_pid to thread_group_leader for clarity.

Update the comment in lookup_task to remove implementation details that
are no longer true and to mention task->signal instead of task->sighand,
as the relevant cpu timer details are all in task->signal.

Ref: 55e8c8eb ("posix-cpu-timers: Store a reference to a pid not a task")
Ref: c0deae8c ("posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

In the work to remove proc_mnt I noticed that we were calling
proc_flush_task now proc_flush_pid possibly multiple times for the same
pid because of how de_thread works.

This is a bare minimal patchset to sort out de_thread, by introducing
exchange_tids and the helper of exchange_tids hlists_swap_heads_rcu.

The actual call of exchange_tids should be slowpath so I have
prioritized readability over getting every last drop of performance.

I have also read through a bunch of the code to see if I could find
anything that would be affected by this change.  Users of
has_group_leader_pid were a good canidates.  But I also looked at other
cases that might have a pid->task->pid transition.  I ignored other
sources of races with de_thread and exec as those are preexisting.

I found a close call with send_signals user of task_active_pid_ns, but
all pids of a thread group are guaranteeds to be in the same pid
namespace so there is not a problem.

I found a few pieces of debugging code that do:

	task = pid_task(pid, PIDTYPE_PID);
	if (task) {
		printk("%u\n", task->pid);
	}

But I can't see how we care if it happens at the wrong moment that
task->pid might not match pid_nr(pid);

Similarly because the code in posix-cpu-timers goes pid->task->pid it
feels like there should be a problem.  But as the code that works with
PIDTYPE_PID is only available within the thread group, and as de_thread
kills all of the other threads before it makes any changes of this
kind the race can not happen.

In short I don't think this change will introduce any regressions.

Eric W. Biederman (2):
      rculist: Add hlists_swap_heads_rcu
      proc: Ensure we see the exit of each process tid exactly once

 fs/exec.c               |  5 +----
 include/linux/pid.h     |  1 +
 include/linux/rculist.h | 21 +++++++++++++++++++++
 kernel/pid.c            | 19 +++++++++++++++++++
 4 files changed, 42 insertions(+), 4 deletions(-)

Link: https://lore.kernel.org/lkml/87sggnajpv.fsf_-_@x220.int.ebiederm.org/Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

When the thread group leader changes during exec and the old leaders
thread is reaped proc_flush_pid will flush the dentries for the entire
process because the leader still has it's original pid.

Fix this by exchanging the pids in an rcu safe manner,
and wrapping the code to do that up in a helper exchange_tids.

When I removed switch_exec_pids and introduced this behavior
in d73d6529 ("[PATCH] pidhash: kill switch_exec_pids") there
really was nothing that cared as flushing happened with
the cached dentry and de_thread flushed both of them on exec.

This lack of fully exchanging pids became a problem a few months later
when I introduced 48e6484d ("[PATCH] proc: Rewrite the proc dentry
flush on exit optimization").  Which overlooked the de_thread case
was no longer swapping pids, and I was looking up proc dentries
by task->pid.

The current behavior isn't properly a bug as everything in proc will
continue to work correctly just a little bit less efficiently.  Fix
this just so there are no little surprise corner cases waiting to bite
people.

-- Oleg points out this could be an issue in next_tgid in proc where
   has_group_leader_pid is called, and reording some of the assignments
   should fix that.

-- Oleg points out this will break the 10 year old hack in __exit_signal.c
>	/*
>	 * This can only happen if the caller is de_thread().
>	 * FIXME: this is the temporary hack, we should teach
>	 * posix-cpu-timers to handle this case correctly.
>	 */
>	if (unlikely(has_group_leader_pid(tsk)))
>		posix_cpu_timers_exit_group(tsk);

The code in next_tgid has been changed to use PIDTYPE_TGID,
and the posix cpu timers code has been fixed so it does not
need the 10 year old hack, so this should be safe to merge
now.

Link: https://lore.kernel.org/lkml/87h7x3ajll.fsf_-_@x220.int.ebiederm.org/Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Fixes: 48e6484d ("[PATCH] proc: Rewrite the proc dentry flush on exit optimization").
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Using the struct pid to refer to two tasks in de_thread was a clever
idea and ultimately too clever, as it has lead to proc_flush_task
being called inconsistently.

To support rectifying this add hlists_swap_heads_rcu.  An hlist
primitive that just swaps the hlist heads of two lists.  This is
exactly what is needed for exchanging the pids of two tasks.

Only consideration of correctness of the code has been given,
as the caller is expected to be a slowpath.

Link: https://lore.kernel.org/lkml/87mu6vajnq.fsf_-_@x220.int.ebiederm.org/Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Combine the pid_task and thes test has_group_leader_pid into a single
dereference by using pid_task(PIDTYPE_TGID).

This makes the code simpler and proof against needing to even think
about any shenanigans that de_thread might get up to.
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Alexey Gladkov <gladkov.alexey@gmail.com> writes:
 Procfs modernization:
 ---------------------
 Historically procfs was always tied to pid namespaces, during pid
 namespace creation we internally create a procfs mount for it. However,
 this has the effect that all new procfs mounts are just a mirror of the
 internal one, any change, any mount option update, any new future
 introduction will propagate to all other procfs mounts that are in the
 same pid namespace.

 This may have solved several use cases in that time. However today we
 face new requirements, and making procfs able to support new private
 instances inside same pid namespace seems a major point. If we want to
 to introduce new features and security mechanisms we have to make sure
 first that we do not break existing usecases. Supporting private procfs
 instances will allow to support new features and behaviour without
 propagating it to all other procfs mounts.

 Today procfs is more of a burden especially to some Embedded, IoT,
 sandbox, container use cases. In user space we are over-mounting null
 or inaccessible files on top to hide files and information. If we want
 to hide pids we have to create PID namespaces otherwise mount options
 propagate to all other proc mounts, changing a mount option value in one
 mount will propagate to all other proc mounts. If we want to introduce
 new features, then they will propagate to all other mounts too, resulting
 either maybe new useful functionality or maybe breaking stuff. We have
 also to note that userspace should not workaround procfs, the kernel
 should just provide a sane simple interface.

 In this regard several developers and maintainers pointed out that
 there are problems with procfs and it has to be modernized:

 "Here's another one: split up and modernize /proc." by Andy Lutomirski [1]

 Discussion about kernel pointer leaks:

 "And yes, as Kees and Daniel mentioned, it's definitely not just dmesg.
 In fact, the primary things tend to be /proc and /sys, not dmesg
 itself." By Linus Torvalds [2]

 Lot of other areas in the kernel and filesystems have been updated to be
 able to support private instances, devpts is one major example [3].

 Which will be used for:

 1) Embedded systems and IoT: usually we have one supervisor for
 apps, we have some lightweight sandbox support, however if we create
 pid namespaces we have to manage all the processes inside too,
 where our goal is to be able to run a bunch of apps each one inside
 its own mount namespace, maybe use network namespaces for vlans
 setups, but right now we only want mount namespaces, without all the
 other complexity. We want procfs to behave more like a real file system,
 and block access to inodes that belong to other users. The 'hidepid=' will
 not work since it is a shared mount option.

 2) Containers, sandboxes and Private instances of file systems - devpts case
 Historically, lot of file systems inside Linux kernel view when instantiated
 were just a mirror of an already created and mounted filesystem. This was the
 case of devpts filesystem, it seems at that time the requirements were to
 optimize things and reuse the same memory, etc. This design used to work but not
 anymore with today's containers, IoT, hostile environments and all the privacy
 challenges that Linux faces.

 In that regards, devpts was updated so that each new mounts is a total
 independent file system by the following patches:

 "devpts: Make each mount of devpts an independent filesystem" by
 Eric W. Biederman [3] [4]

 3) Linux Security Modules have multiple ptrace paths inside some
 subsystems, however inside procfs, the implementation does not guarantee
 that the ptrace() check which triggers the security_ptrace_check() hook
 will always run. We have the 'hidepid' mount option that can be used to
 force the ptrace_may_access() check inside has_pid_permissions() to run.
 The problem is that 'hidepid' is per pid namespace and not attached to
 the mount point, any remount or modification of 'hidepid' will propagate
 to all other procfs mounts.

 This also does not allow to support Yama LSM easily in desktop and user
 sessions. Yama ptrace scope which restricts ptrace and some other
 syscalls to be allowed only on inferiors, can be updated to have a
 per-task context, where the context will be inherited during fork(),
 clone() and preserved across execve(). If we support multiple private
 procfs instances, then we may force the ptrace_may_access() on
 /proc/<pids>/ to always run inside that new procfs instances. This will
 allow to specifiy on user sessions if we should populate procfs with
 pids that the user can ptrace or not.

 By using Yama ptrace scope, some restricted users will only be able to see
 inferiors inside /proc, they won't even be able to see their other
 processes. Some software like Chromium, Firefox's crash handler, Wine
 and others are already using Yama to restrict which processes can be
 ptracable. With this change this will give the possibility to restrict
 /proc/<pids>/ but more importantly this will give desktop users a
 generic and usuable way to specifiy which users should see all processes
 and which user can not.

 Side notes:

 * This covers the lack of seccomp where it is not able to parse
 arguments, it is easy to install a seccomp filter on direct syscalls
 that operate on pids, however /proc/<pid>/ is a Linux ABI using
 filesystem syscalls. With this change all LSMs should be able to analyze
 open/read/write/close... on /proc/<pid>/

 4) This will allow to implement new features either in kernel or
 userspace without having to worry about procfs.
 In containers, sandboxes, etc we have workarounds to hide some /proc
 inodes, this should be supported natively without doing extra complex
 work, the kernel should be able to support sane options that work with
 today and future Linux use cases.

 5) Creation of new superblock with all procfs options for each procfs
 mount will fix the ignoring of mount options. The problem is that the
 second mount of procfs in the same pid namespace ignores the mount
 options. The mount options are ignored without error until procfs is
 remounted.

 Before:

 proc /proc proc rw,relatime,hidepid=2 0 0

 mount("proc", "/tmp/proc", "proc", 0, "hidepid=1") = 0
 +++ exited with 0 +++

 proc /proc proc rw,relatime,hidepid=2 0 0
 proc /tmp/proc proc rw,relatime,hidepid=2 0 0

 proc /proc proc rw,relatime,hidepid=1 0 0
 proc /tmp/proc proc rw,relatime,hidepid=1 0 0

 After:

 proc /proc proc rw,relatime,hidepid=ptraceable 0 0

 proc /proc proc rw,relatime,hidepid=ptraceable 0 0
 proc /tmp/proc proc rw,relatime,hidepid=invisible 0 0

 Introduced changes:
 -------------------
 Each mount of procfs creates a separate procfs instance with its own
 mount options.

 This series adds few new mount options:

 * New 'hidepid=ptraceable' or 'hidepid=4' mount option to show only ptraceable
 processes in the procfs. This allows to support lightweight sandboxes in
 Embedded Linux, also solves the case for LSM where now with this mount option,
 we make sure that they have a ptrace path in procfs.

 * 'subset=pid' that allows to hide non-pid inodes from procfs. It can be used
 in containers and sandboxes, as these are already trying to hide and block
 access to procfs inodes anyway.

 ChangeLog:
 ----------
 * Rebase on top of v5.7-rc1.
 * Fix a resource leak if proc is not mounted or if proc is simply reconfigured.
 * Add few selftests.

 * After a discussion with Eric W. Biederman, the numerical values for hidepid
   parameter have been removed from uapi.
 * Remove proc_self and proc_thread_self from the pid_namespace struct.
 * I took into account the comment of Kees Cook.
 * Update Reviewed-by tags.

 * 'subset=pidfs' renamed to 'subset=pid' as suggested by Alexey Dobriyan.
 * Include Reviewed-by tags.

 * Rebase on top of Eric W. Biederman's procfs changes.
 * Add human readable values of 'hidepid' as suggested by Andy Lutomirski.

 * Started using RCU lock to clean dcache entries as suggested by Linus Torvalds.

 * 'pidonly=1' renamed to 'subset=pidfs' as suggested by Alexey Dobriyan.
 * HIDEPID_* moved to uapi/ as they are user interface to mount().
   Suggested-by Alexey Dobriyan <adobriyan@gmail.com>

 * 'hidepid=' and 'gid=' mount options are moved from pid namespace to superblock.
 * 'newinstance' mount option removed as suggested by Eric W. Biederman.
    Mount of procfs always creates a new instance.
 * 'limit_pids' renamed to 'hidepid=3'.
 * I took into account the comment of Linus Torvalds [7].
 * Documentation added.

 * Fixed a bug that caused a problem with the Fedora boot.
 * The 'pidonly' option is visible among the mount options.

 * Renamed mount options to 'newinstance' and 'pids='
Suggested-by: Andy Lutomirski <luto@kernel.org>
 * Fixed order of commit, Suggested-by: Andy Lutomirski <luto@kernel.org>
 * Many bug fixes.

 * Removed 'unshared' mount option and replaced it with 'limit_pids'
    which is attached to the current procfs mount.
    Suggested-by Andy Lutomirski <luto@kernel.org>
 * Do not fill dcache with pid entries that we can not ptrace.
 * Many bug fixes.

 References:
 -----------
 [1] https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2017-January/004215.html
 [2] http://www.openwall.com/lists/kernel-hardening/2017/10/05/5
 [3] https://lwn.net/Articles/689539/
 [4] http://lxr.free-electrons.com/source/Documentation/filesystems/devpts.txt?v=3.14
 [5] https://lkml.org/lkml/2017/5/2/407
 [6] https://lkml.org/lkml/2017/5/3/357
 [7] https://lkml.org/lkml/2018/5/11/505

 Alexey Gladkov (7):
   proc: rename struct proc_fs_info to proc_fs_opts
   proc: allow to mount many instances of proc in one pid namespace
   proc: instantiate only pids that we can ptrace on 'hidepid=4' mount
     option
   proc: add option to mount only a pids subset
   docs: proc: add documentation for "hidepid=4" and "subset=pid" options
     and new mount behavior
  proc: use human-readable values for hidepid
   proc: use named enums for better readability

  Documentation/filesystems/proc.rst            |  92 +++++++++---
  fs/proc/base.c                                |  48 +++++--
  fs/proc/generic.c                             |   9 ++
  fs/proc/inode.c                               |  30 +++-
  fs/proc/root.c                                | 131 +++++++++++++-----
  fs/proc/self.c                                |   6 +-
  fs/proc/thread_self.c                         |   6 +-
  fs/proc_namespace.c                           |  14 +-
  include/linux/pid_namespace.h                 |  12 --
  include/linux/proc_fs.h                       |  30 +++-
  tools/testing/selftests/proc/.gitignore       |   2 +
  tools/testing/selftests/proc/Makefile         |   2 +
  .../selftests/proc/proc-fsconfig-hidepid.c    |  50 +++++++
  .../selftests/proc/proc-multiple-procfs.c     |  48 +++++++
  14 files changed, 384 insertions(+), 96 deletions(-)
  create mode 100644 tools/testing/selftests/proc/proc-fsconfig-hidepid.c
  create mode 100644 tools/testing/selftests/proc/proc-multiple-procfs.c

Link: https://lore.kernel.org/lkml/20200419141057.621356-1-gladkov.alexey@gmail.com/Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

To get pid_namespace from the procfs superblock should be used a special
helper. This will avoid errors when s_fs_info will change the type.

Link: https://lore.kernel.org/lkml/20200423200316.164518-3-gladkov.alexey@gmail.com/
Link: https://lore.kernel.org/lkml/20200423112858.95820-1-gladkov.alexey@gmail.com/
Link: https://lore.kernel.org/lkml/06B50A1C-406F-4057-BFA8-3A7729EA7469@lca.pw/Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Oleg pointed out that in the unlikely event the kernel is compiled
with CONFIG_PROC_FS unset that release_task will now leak the pid.

Move the put_pid out of proc_flush_pid into release_task to fix this
and to guarantee I don't make that mistake again.

When possible it makes sense to keep get and put in the same function
so it can easily been seen how they pair up.

Fixes: 7bc3e6e5 ("proc: Use a list of inodes to flush from proc")
Reported-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

The hidepid parameter values are becoming more and more and it becomes
difficult to remember what each new magic number means.

Backward compatibility is preserved since it is possible to specify
numerical value for the hidepid parameter. This does not break the
fsconfig since it is not possible to specify a numerical value through
it. All numeric values are converted to a string. The type
FSCONFIG_SET_BINARY cannot be used to indicate a numerical value.

Selftest has been added to verify this behavior.
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

This allows to hide all files and directories in the procfs that are not
related to tasks.
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

If "hidepid=4" mount option is set then do not instantiate pids that
we can not ptrace. "hidepid=4" means that procfs should only contain
pids that the caller can ptrace.
Signed-off-by: Djalal Harouni <tixxdz@gmail.com>
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

This patch allows to have multiple procfs instances inside the
same pid namespace. The aim here is lightweight sandboxes, and to allow
that we have to modernize procfs internals.

1) The main aim of this work is to have on embedded systems one
supervisor for apps. Right now we have some lightweight sandbox support,
however if we create pid namespacess we have to manages all the
processes inside too, where our goal is to be able to run a bunch of
apps each one inside its own mount namespace without being able to
notice each other. We only want to use mount namespaces, and we want
procfs to behave more like a real mount point.

2) Linux Security Modules have multiple ptrace paths inside some
subsystems, however inside procfs, the implementation does not guarantee
that the ptrace() check which triggers the security_ptrace_check() hook
will always run. We have the 'hidepid' mount option that can be used to
force the ptrace_may_access() check inside has_pid_permissions() to run.
The problem is that 'hidepid' is per pid namespace and not attached to
the mount point, any remount or modification of 'hidepid' will propagate
to all other procfs mounts.

This also does not allow to support Yama LSM easily in desktop and user
sessions. Yama ptrace scope which restricts ptrace and some other
syscalls to be allowed only on inferiors, can be updated to have a
per-task context, where the context will be inherited during fork(),
clone() and preserved across execve(). If we support multiple private
procfs instances, then we may force the ptrace_may_access() on
/proc/<pids>/ to always run inside that new procfs instances. This will
allow to specifiy on user sessions if we should populate procfs with
pids that the user can ptrace or not.

By using Yama ptrace scope, some restricted users will only be able to see
inferiors inside /proc, they won't even be able to see their other
processes. Some software like Chromium, Firefox's crash handler, Wine
and others are already using Yama to restrict which processes can be
ptracable. With this change this will give the possibility to restrict
/proc/<pids>/ but more importantly this will give desktop users a
generic and usuable way to specifiy which users should see all processes
and which users can not.

Side notes:
* This covers the lack of seccomp where it is not able to parse
arguments, it is easy to install a seccomp filter on direct syscalls
that operate on pids, however /proc/<pid>/ is a Linux ABI using
filesystem syscalls. With this change LSMs should be able to analyze
open/read/write/close...

In the new patch set version I removed the 'newinstance' option
as suggested by Eric W. Biederman.

Selftest has been added to verify new behavior.
Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Christof Meerwald <cmeerw@cmeerw.org> writes:
> Hi,
>
> this is probably related to commit
> 7a0cf094 (signal: Correct namespace
> fixups of si_pid and si_uid).
>
> With a 5.6.5 kernel I am seeing SIGCHLD signals that don't include a
> properly set si_pid field - this seems to happen for multi-threaded
> child processes.
>
> A simple test program (based on the sample from the signalfd man page):
>
> #include <sys/signalfd.h>
> #include <signal.h>
> #include <unistd.h>
> #include <spawn.h>
> #include <stdlib.h>
> #include <stdio.h>
>
> #define handle_error(msg) \
>     do { perror(msg); exit(EXIT_FAILURE); } while (0)
>
> int main(int argc, char *argv[])
> {
>   sigset_t mask;
>   int sfd;
>   struct signalfd_siginfo fdsi;
>   ssize_t s;
>
>   sigemptyset(&mask);
>   sigaddset(&mask, SIGCHLD);
>
>   if (sigprocmask(SIG_BLOCK, &mask, NULL) == -1)
>     handle_error("sigprocmask");
>
>   pid_t chldpid;
>   char *chldargv[] = { "./sfdclient", NULL };
>   posix_spawn(&chldpid, "./sfdclient", NULL, NULL, chldargv, NULL);
>
>   sfd = signalfd(-1, &mask, 0);
>   if (sfd == -1)
>     handle_error("signalfd");
>
>   for (;;) {
>     s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
>     if (s != sizeof(struct signalfd_siginfo))
>       handle_error("read");
>
>     if (fdsi.ssi_signo == SIGCHLD) {
>       printf("Got SIGCHLD %d %d %d %d\n",
>           fdsi.ssi_status, fdsi.ssi_code,
>           fdsi.ssi_uid, fdsi.ssi_pid);
>       return 0;
>     } else {
>       printf("Read unexpected signal\n");
>     }
>   }
> }
>
>
> and a multi-threaded client to test with:
>
> #include <unistd.h>
> #include <pthread.h>
>
> void *f(void *arg)
> {
>   sleep(100);
> }
>
> int main()
> {
>   pthread_t t[8];
>
>   for (int i = 0; i != 8; ++i)
>   {
>     pthread_create(&t[i], NULL, f, NULL);
>   }
> }
>
> I tried to do a bit of debugging and what seems to be happening is
> that
>
>   /* From an ancestor pid namespace? */
>   if (!task_pid_nr_ns(current, task_active_pid_ns(t))) {
>
> fails inside task_pid_nr_ns because the check for "pid_alive" fails.
>
> This code seems to be called from do_notify_parent and there we
> actually have "tsk != current" (I am assuming both are threads of the
> current process?)

I instrumented the code with a warning and received the following backtrace:
> WARNING: CPU: 0 PID: 777 at kernel/pid.c:501 __task_pid_nr_ns.cold.6+0xc/0x15
> Modules linked in:
> CPU: 0 PID: 777 Comm: sfdclient Not tainted 5.7.0-rc1userns+ #2924
> Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
> RIP: 0010:__task_pid_nr_ns.cold.6+0xc/0x15
> Code: ff 66 90 48 83 ec 08 89 7c 24 04 48 8d 7e 08 48 8d 74 24 04 e8 9a b6 44 00 48 83 c4 08 c3 48 c7 c7 59 9f ac 82 e8 c2 c4 04 00 <0f> 0b e9 3fd
> RSP: 0018:ffffc9000042fbf8 EFLAGS: 00010046
> RAX: 000000000000000c RBX: 0000000000000000 RCX: ffffc9000042faf4
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81193d29
> RBP: ffffc9000042fc18 R08: 0000000000000000 R09: 0000000000000001
> R10: 000000100f938416 R11: 0000000000000309 R12: ffff8880b941c140
> R13: 0000000000000000 R14: 0000000000000000 R15: ffff8880b941c140
> FS:  0000000000000000(0000) GS:ffff8880bca00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f2e8c0a32e0 CR3: 0000000002e10000 CR4: 00000000000006f0
> Call Trace:
>  send_signal+0x1c8/0x310
>  do_notify_parent+0x50f/0x550
>  release_task.part.21+0x4fd/0x620
>  do_exit+0x6f6/0xaf0
>  do_group_exit+0x42/0xb0
>  get_signal+0x13b/0xbb0
>  do_signal+0x2b/0x670
>  ? __audit_syscall_exit+0x24d/0x2b0
>  ? rcu_read_lock_sched_held+0x4d/0x60
>  ? kfree+0x24c/0x2b0
>  do_syscall_64+0x176/0x640
>  ? trace_hardirqs_off_thunk+0x1a/0x1c
>  entry_SYSCALL_64_after_hwframe+0x49/0xb3

The immediate problem is as Christof noticed that "pid_alive(current) == false".
This happens because do_notify_parent is called from the last thread to exit
in a process after that thread has been reaped.

The bigger issue is that do_notify_parent can be called from any
process that manages to wait on a thread of a multi-threaded process
from wait_task_zombie.  So any logic based upon current for
do_notify_parent is just nonsense, as current can be pretty much
anything.

So change do_notify_parent to call __send_signal directly.

Inspecting the code it appears this problem has existed since the pid
namespace support started handling this case in 2.6.30.  This fix only
backports to 7a0cf094 ("signal: Correct namespace fixups of si_pid and si_uid")
where the problem logic was moved out of __send_signal and into send_signal.

Cc: stable@vger.kernel.org
Fixes: 6588c1e3 ("signals: SI_USER: Masquerade si_pid when crossing pid ns boundary")
Ref: 921cf9f6 ("signals: protect cinit from unblocked SIG_DFL signals")
Link: https://lore.kernel.org/lkml/20200419201336.GI22017@edge.cmeerw.net/Reported-by: Christof Meerwald <cmeerw@cmeerw.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

syzbot writes:
> KASAN: use-after-free Read in dput (2)
>
> proc_fill_super: allocate dentry failed
> ==================================================================
> BUG: KASAN: use-after-free in fast_dput fs/dcache.c:727 [inline]
> BUG: KASAN: use-after-free in dput+0x53e/0xdf0 fs/dcache.c:846
> Read of size 4 at addr ffff88808a618cf0 by task syz-executor.0/8426
>
> CPU: 0 PID: 8426 Comm: syz-executor.0 Not tainted 5.6.0-next-20200412-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:77 [inline]
>  dump_stack+0x188/0x20d lib/dump_stack.c:118
>  print_address_description.constprop.0.cold+0xd3/0x315 mm/kasan/report.c:382
>  __kasan_report.cold+0x35/0x4d mm/kasan/report.c:511
>  kasan_report+0x33/0x50 mm/kasan/common.c:625
>  fast_dput fs/dcache.c:727 [inline]
>  dput+0x53e/0xdf0 fs/dcache.c:846
>  proc_kill_sb+0x73/0xf0 fs/proc/root.c:195
>  deactivate_locked_super+0x8c/0xf0 fs/super.c:335
>  vfs_get_super+0x258/0x2d0 fs/super.c:1212
>  vfs_get_tree+0x89/0x2f0 fs/super.c:1547
>  do_new_mount fs/namespace.c:2813 [inline]
>  do_mount+0x1306/0x1b30 fs/namespace.c:3138
>  __do_sys_mount fs/namespace.c:3347 [inline]
>  __se_sys_mount fs/namespace.c:3324 [inline]
>  __x64_sys_mount+0x18f/0x230 fs/namespace.c:3324
>  do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
>  entry_SYSCALL_64_after_hwframe+0x49/0xb3
> RIP: 0033:0x45c889
> Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> RSP: 002b:00007ffc1930ec48 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 0000000001324914 RCX: 000000000045c889
> RDX: 0000000020000140 RSI: 0000000020000040 RDI: 0000000000000000
> RBP: 000000000076bf00 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
> R13: 0000000000000749 R14: 00000000004ca15a R15: 0000000000000013

Looking at the code now that it the internal mount of proc is no
longer used it is possible to unmount proc.   If proc is unmounted
the fields of the pid namespace that were used for filesystem
specific state are not reinitialized.

Which means that proc_self and proc_thread_self can be pointers to
already freed dentries.

The reported user after free appears to be from mounting and
unmounting proc followed by mounting proc again and using error
injection to cause the new root dentry allocation to fail.  This in
turn results in proc_kill_sb running with proc_self and
proc_thread_self still retaining their values from the previous mount
of proc.  Then calling dput on either proc_self of proc_thread_self
will result in double put.  Which KASAN sees as a use after free.

Solve this by always reinitializing the filesystem state stored
in the struct pid_namespace, when proc is unmounted.

Reported-by: syzbot+72868dd424eb66c6b95f@syzkaller.appspotmail.com
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Fixes: 69879c01 ("proc: Remove the now unnecessary internal mount of proc")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

This sorts the actual field names too, potentially causing even more
chaos and confusion at merge time if you have edited the MAINTAINERS
file.  But the end result is a more consistent layout, and hopefully
it's a one-time pain minimized by doing this just before the -rc1
release.

This was entirely scripted:

  ./scripts/parse-maintainers.pl --input=MAINTAINERS --output=MAINTAINERS --order
Requested-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

They are all supposed to be sorted, but people who add new entries don't
always know the alphabet.  Plus sometimes the entry names get edited,
and people don't then re-order the entry.

Let's see how painful this will be for merging purposes (the MAINTAINERS
file is often edited in various different trees), but Joe claims there's
relatively few patches in -next that touch this, and doing it just
before -rc1 is likely the best time.  Fingers crossed.

This was scripted with

  /scripts/parse-maintainers.pl --input=MAINTAINERS --output=MAINTAINERS

but then I also ended up manually upper-casing a few entry names that
stood out when looking at the end result.
Requested-by: Joe Perches <joe@perches.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Pull x86 fixes from Thomas Gleixner:
 "A set of three patches to fix the fallout of the newly added split
  lock detection feature.

  It addressed the case where a KVM guest triggers a split lock #AC and
  KVM reinjects it into the guest which is not prepared to handle it.

  Add proper sanity checks which prevent the unconditional injection
  into the guest and handles the #AC on the host side in the same way as
  user space detections are handled. Depending on the detection mode it
  either warns and disables detection for the task or kills the task if
  the mode is set to fatal"

* tag 'x86-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  KVM: VMX: Extend VMXs #AC interceptor to handle split lock #AC in guest
  KVM: x86: Emulate split-lock access as a write in emulator
  x86/split_lock: Provide handle_guest_split_lock()

Pull time(keeping) updates from Thomas Gleixner:

 - Fix the time_for_children symlink in /proc/$PID/ so it properly
   reflects that it part of the 'time' namespace

 - Add the missing userns limit for the allowed number of time
   namespaces, which was half defined but the actual array member was
   not added. This went unnoticed as the array has an exessive empty
   member at the end but introduced a user visible regression as the
   output was corrupted.

 - Prevent further silent ucount corruption by adding a BUILD_BUG_ON()
   to catch half updated data.

* tag 'timers-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  ucount: Make sure ucounts in /proc/sys/user don't regress again
  time/namespace: Add max_time_namespaces ucount
  time/namespace: Fix time_for_children symlink

Pull scheduler fixes/updates from Thomas Gleixner:

 - Deduplicate the average computations in the scheduler core and the
   fair class code.

 - Fix a raise between runtime distribution and assignement which can
   cause exceeding the quota by up to 70%.

 - Prevent negative results in the imbalanace calculation

 - Remove a stale warning in the workqueue code which can be triggered
   since the call site was moved out of preempt disabled code. It's a
   false positive.

 - Deduplicate the print macros for procfs

 - Add the ucmap values to the SCHED_DEBUG procfs output for completness

* tag 'sched-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  sched/debug: Add task uclamp values to SCHED_DEBUG procfs
  sched/debug: Factor out printing formats into common macros
  sched/debug: Remove redundant macro define
  sched/core: Remove unused rq::last_load_update_tick
  workqueue: Remove the warning in wq_worker_sleeping()
  sched/fair: Fix negative imbalance in imbalance calculation
  sched/fair: Fix race between runtime distribution and assignment
  sched/fair: Align rq->avg_idle and rq->avg_scan_cost

Pull perf fixes from Thomas Gleixner:
 "Three fixes/updates for perf:

   - Fix the perf event cgroup tracking which tries to track the cgroup
     even for disabled events.

   - Add Ice Lake server support for uncore events

   - Disable pagefaults when retrieving the physical address in the
     sampling code"

* tag 'perf-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  perf/core: Disable page faults when getting phys address
  perf/x86/intel/uncore: Add Ice Lake server uncore support
  perf/cgroup: Correct indirection in perf_less_group_idx()
  perf/core: Fix event cgroup tracking

Pull locking fixes from Thomas Gleixner:
 "Three small fixes/updates for the locking core code:

   - Plug a task struct reference leak in the percpu rswem
     implementation.

   - Document the refcount interaction with PID_MAX_LIMIT

   - Improve the 'invalid wait context' data dump in lockdep so it
     contains all information which is required to decode the problem"

* tag 'locking-urgent-2020-04-12' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/lockdep: Improve 'invalid wait context' splat
  locking/refcount: Document interaction with PID_MAX_LIMIT
  locking/percpu-rwsem: Fix a task_struct refcount

Pull cifs fixes from Steve French:
 "Ten cifs/smb fixes:

   - five RDMA (smbdirect) related fixes

   - add experimental support for swap over SMB3 mounts

   - also a fix which improves performance of signed connections"

* tag '5.7-rc-smb3-fixes-part2' of git://git.samba.org/sfrench/cifs-2.6:
  smb3: enable swap on SMB3 mounts
  smb3: change noisy error message to FYI
  smb3: smbdirect support can be configured by default
  cifs: smbd: Do not schedule work to send immediate packet on every receive
  cifs: smbd: Properly process errors on ib_post_send
  cifs: Allocate crypto structures on the fly for calculating signatures of incoming packets
  cifs: smbd: Update receive credits before sending and deal with credits roll back on failure before sending
  cifs: smbd: Check send queue size before posting a send
  cifs: smbd: Merge code to track pending packets
  cifs: ignore cached share root handle closing errors

Pull NFS client bugfix from Trond Myklebust:
 "Fix an RCU read lock leakage in pnfs_alloc_ds_commits_list()"

* tag 'nfs-for-5.7-2' of git://git.linux-nfs.org/projects/trondmy/linux-nfs:
  pNFS: Fix RCU lock leakage

Pull nios2 updates from Ley Foon Tan:

 - Remove nios2-dev@lists.rocketboards.org from MAINTAINERS

 - remove 'resetvalue' property

 - rename 'altr,gpio-bank-width' -> 'altr,ngpio'

 - enable the common clk subsystem on Nios2

* tag 'nios2-v5.7-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/lftan/nios2:
  MAINTAINERS: Remove nios2-dev@lists.rocketboards.org
  arch: nios2: remove 'resetvalue' property
  arch: nios2: rename 'altr,gpio-bank-width' -> 'altr,ngpio'
  arch: nios2: Enable the common clk subsystem on Nios2