Commit 1c8d4225 authored by Jann Horn's avatar Jann Horn Committed by Ben Hutchings

ptrace: use fsuid, fsgid, effective creds for fs access checks

commit caaee623 upstream.

By checking the effective credentials instead of the real UID / permitted
capabilities, ensure that the calling process actually intended to use its
credentials.

To ensure that all ptrace checks use the correct caller credentials (e.g.
in case out-of-tree code or newly added code omits the PTRACE_MODE_*CREDS
flag), use two new flags and require one of them to be set.

The problem was that when a privileged task had temporarily dropped its
privileges, e.g.  by calling setreuid(0, user_uid), with the intent to
perform following syscalls with the credentials of a user, it still passed
ptrace access checks that the user would not be able to pass.

While an attacker should not be able to convince the privileged task to
perform a ptrace() syscall, this is a problem because the ptrace access
check is reused for things in procfs.

In particular, the following somewhat interesting procfs entries only rely
on ptrace access checks:

 /proc/$pid/stat - uses the check for determining whether pointers
     should be visible, useful for bypassing ASLR
 /proc/$pid/maps - also useful for bypassing ASLR
 /proc/$pid/cwd - useful for gaining access to restricted
     directories that contain files with lax permissions, e.g. in
     this scenario:
     lrwxrwxrwx root root /proc/13020/cwd -> /root/foobar
     drwx------ root root /root
     drwxr-xr-x root root /root/foobar
     -rw-r--r-- root root /root/foobar/secret

Therefore, on a system where a root-owned mode 6755 binary changes its
effective credentials as described and then dumps a user-specified file,
this could be used by an attacker to reveal the memory layout of root's
processes or reveal the contents of files he is not allowed to access
(through /proc/$pid/cwd).

[akpm@linux-foundation.org: fix warning]
Signed-off-by: default avatarJann Horn <jann@thejh.net>
Acked-by: default avatarKees Cook <keescook@chromium.org>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: "Serge E. Hallyn" <serge.hallyn@ubuntu.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2:
 - Drop changes to kcmp, procfs map_files, procfs has_pid_permissions()
 - Keep using uid_t, gid_t and == operator for IDs
 - Adjust context]
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 33bab922
...@@ -380,7 +380,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns, ...@@ -380,7 +380,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
state = *get_task_state(task); state = *get_task_state(task);
vsize = eip = esp = 0; vsize = eip = esp = 0;
permitted = ptrace_may_access(task, PTRACE_MODE_READ); permitted = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
mm = get_task_mm(task); mm = get_task_mm(task);
if (mm) { if (mm) {
vsize = task_vsize(mm); vsize = task_vsize(mm);
......
...@@ -216,7 +216,7 @@ static struct mm_struct *mm_access(struct task_struct *task, unsigned int mode) ...@@ -216,7 +216,7 @@ static struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
struct mm_struct *mm_for_maps(struct task_struct *task) struct mm_struct *mm_for_maps(struct task_struct *task)
{ {
return mm_access(task, PTRACE_MODE_READ); return mm_access(task, PTRACE_MODE_READ_FSCREDS);
} }
static int proc_pid_cmdline(struct task_struct *task, char * buffer) static int proc_pid_cmdline(struct task_struct *task, char * buffer)
...@@ -288,7 +288,7 @@ static int proc_pid_wchan(struct task_struct *task, char *buffer) ...@@ -288,7 +288,7 @@ static int proc_pid_wchan(struct task_struct *task, char *buffer)
wchan = get_wchan(task); wchan = get_wchan(task);
if (lookup_symbol_name(wchan, symname) < 0) if (lookup_symbol_name(wchan, symname) < 0)
if (!ptrace_may_access(task, PTRACE_MODE_READ)) if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
return 0; return 0;
else else
return sprintf(buffer, "%lu", wchan); return sprintf(buffer, "%lu", wchan);
...@@ -302,7 +302,7 @@ static int lock_trace(struct task_struct *task) ...@@ -302,7 +302,7 @@ static int lock_trace(struct task_struct *task)
int err = mutex_lock_killable(&task->signal->cred_guard_mutex); int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
if (err) if (err)
return err; return err;
if (!ptrace_may_access(task, PTRACE_MODE_ATTACH)) { if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_FSCREDS)) {
mutex_unlock(&task->signal->cred_guard_mutex); mutex_unlock(&task->signal->cred_guard_mutex);
return -EPERM; return -EPERM;
} }
...@@ -544,7 +544,7 @@ static int proc_fd_access_allowed(struct inode *inode) ...@@ -544,7 +544,7 @@ static int proc_fd_access_allowed(struct inode *inode)
*/ */
task = get_proc_task(inode); task = get_proc_task(inode);
if (task) { if (task) {
allowed = ptrace_may_access(task, PTRACE_MODE_READ); allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
put_task_struct(task); put_task_struct(task);
} }
return allowed; return allowed;
...@@ -769,7 +769,7 @@ static int mem_open(struct inode* inode, struct file* file) ...@@ -769,7 +769,7 @@ static int mem_open(struct inode* inode, struct file* file)
if (!task) if (!task)
return -ESRCH; return -ESRCH;
mm = mm_access(task, PTRACE_MODE_ATTACH); mm = mm_access(task, PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS);
put_task_struct(task); put_task_struct(task);
if (IS_ERR(mm)) if (IS_ERR(mm))
...@@ -2627,7 +2627,7 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole) ...@@ -2627,7 +2627,7 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
if (result) if (result)
return result; return result;
if (!ptrace_may_access(task, PTRACE_MODE_READ)) { if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
result = -EACCES; result = -EACCES;
goto out_unlock; goto out_unlock;
} }
......
...@@ -91,7 +91,7 @@ static int proc_ns_dir_readdir(struct file *filp, void *dirent, ...@@ -91,7 +91,7 @@ static int proc_ns_dir_readdir(struct file *filp, void *dirent,
goto out_no_task; goto out_no_task;
ret = -EPERM; ret = -EPERM;
if (!ptrace_may_access(task, PTRACE_MODE_READ)) if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out; goto out;
ret = 0; ret = 0;
...@@ -154,7 +154,7 @@ static struct dentry *proc_ns_dir_lookup(struct inode *dir, ...@@ -154,7 +154,7 @@ static struct dentry *proc_ns_dir_lookup(struct inode *dir,
goto out_no_task; goto out_no_task;
error = ERR_PTR(-EPERM); error = ERR_PTR(-EPERM);
if (!ptrace_may_access(task, PTRACE_MODE_READ)) if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
goto out; goto out;
last = &ns_entries[ARRAY_SIZE(ns_entries) - 1]; last = &ns_entries[ARRAY_SIZE(ns_entries) - 1];
......
...@@ -130,9 +130,31 @@ extern void __ptrace_unlink(struct task_struct *child); ...@@ -130,9 +130,31 @@ extern void __ptrace_unlink(struct task_struct *child);
extern void exit_ptrace(struct task_struct *tracer); extern void exit_ptrace(struct task_struct *tracer);
#define PTRACE_MODE_READ 1 #define PTRACE_MODE_READ 1
#define PTRACE_MODE_ATTACH 2 #define PTRACE_MODE_ATTACH 2
#define PTRACE_MODE_FSCREDS 0x08
#define PTRACE_MODE_REALCREDS 0x10
/* shorthands for READ/ATTACH and FSCREDS/REALCREDS combinations */
#define PTRACE_MODE_READ_FSCREDS (PTRACE_MODE_READ | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_READ_REALCREDS (PTRACE_MODE_READ | PTRACE_MODE_REALCREDS)
#define PTRACE_MODE_ATTACH_FSCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_FSCREDS)
#define PTRACE_MODE_ATTACH_REALCREDS (PTRACE_MODE_ATTACH | PTRACE_MODE_REALCREDS)
/* Returns 0 on success, -errno on denial. */ /* Returns 0 on success, -errno on denial. */
extern int __ptrace_may_access(struct task_struct *task, unsigned int mode); extern int __ptrace_may_access(struct task_struct *task, unsigned int mode);
/* Returns true on success, false on denial. */ /**
* ptrace_may_access - check whether the caller is permitted to access
* a target task.
* @task: target task
* @mode: selects type of access and caller credentials
*
* Returns true on success, false on denial.
*
* One of the flags PTRACE_MODE_FSCREDS and PTRACE_MODE_REALCREDS must
* be set in @mode to specify whether the access was requested through
* a filesystem syscall (should use effective capabilities and fsuid
* of the caller) or through an explicit syscall such as
* process_vm_writev or ptrace (and should use the real credentials).
*/
extern bool ptrace_may_access(struct task_struct *task, unsigned int mode); extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
static inline int ptrace_reparented(struct task_struct *child) static inline int ptrace_reparented(struct task_struct *child)
......
...@@ -3004,7 +3004,7 @@ find_lively_task_by_vpid(pid_t vpid) ...@@ -3004,7 +3004,7 @@ find_lively_task_by_vpid(pid_t vpid)
/* Reuse ptrace permission checks for now. */ /* Reuse ptrace permission checks for now. */
err = -EACCES; err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ)) if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS))
goto errout; goto errout;
return task; return task;
......
...@@ -2627,7 +2627,7 @@ SYSCALL_DEFINE3(get_robust_list, int, pid, ...@@ -2627,7 +2627,7 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
} }
ret = -EPERM; ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ)) if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock; goto err_unlock;
head = p->robust_list; head = p->robust_list;
......
...@@ -154,7 +154,7 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr, ...@@ -154,7 +154,7 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr,
} }
ret = -EPERM; ret = -EPERM;
if (!ptrace_may_access(p, PTRACE_MODE_READ)) if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
goto err_unlock; goto err_unlock;
head = p->compat_robust_list; head = p->compat_robust_list;
......
...@@ -220,6 +220,14 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state) ...@@ -220,6 +220,14 @@ int ptrace_check_attach(struct task_struct *child, bool ignore_state)
int __ptrace_may_access(struct task_struct *task, unsigned int mode) int __ptrace_may_access(struct task_struct *task, unsigned int mode)
{ {
const struct cred *cred = current_cred(), *tcred; const struct cred *cred = current_cred(), *tcred;
int dumpable = 0;
uid_t caller_uid;
gid_t caller_gid;
if (!(mode & PTRACE_MODE_FSCREDS) == !(mode & PTRACE_MODE_REALCREDS)) {
WARN(1, "denying ptrace access check without PTRACE_MODE_*CREDS\n");
return -EPERM;
}
/* May we inspect the given task? /* May we inspect the given task?
* This check is used both for attaching with ptrace * This check is used both for attaching with ptrace
...@@ -229,19 +237,34 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode) ...@@ -229,19 +237,34 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
* because setting up the necessary parent/child relationship * because setting up the necessary parent/child relationship
* or halting the specified task is impossible. * or halting the specified task is impossible.
*/ */
int dumpable = 0;
/* Don't let security modules deny introspection */ /* Don't let security modules deny introspection */
if (same_thread_group(task, current)) if (same_thread_group(task, current))
return 0; return 0;
rcu_read_lock(); rcu_read_lock();
if (mode & PTRACE_MODE_FSCREDS) {
caller_uid = cred->fsuid;
caller_gid = cred->fsgid;
} else {
/*
* Using the euid would make more sense here, but something
* in userland might rely on the old behavior, and this
* shouldn't be a security problem since
* PTRACE_MODE_REALCREDS implies that the caller explicitly
* used a syscall that requests access to another process
* (and not a filesystem syscall to procfs).
*/
caller_uid = cred->uid;
caller_gid = cred->gid;
}
tcred = __task_cred(task); tcred = __task_cred(task);
if (cred->user->user_ns == tcred->user->user_ns && if (cred->user->user_ns == tcred->user->user_ns &&
(cred->uid == tcred->euid && (caller_uid == tcred->euid &&
cred->uid == tcred->suid && caller_uid == tcred->suid &&
cred->uid == tcred->uid && caller_uid == tcred->uid &&
cred->gid == tcred->egid && caller_gid == tcred->egid &&
cred->gid == tcred->sgid && caller_gid == tcred->sgid &&
cred->gid == tcred->gid)) caller_gid == tcred->gid))
goto ok; goto ok;
if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE)) if (ns_capable(tcred->user->user_ns, CAP_SYS_PTRACE))
goto ok; goto ok;
...@@ -308,7 +331,7 @@ static int ptrace_attach(struct task_struct *task, long request, ...@@ -308,7 +331,7 @@ static int ptrace_attach(struct task_struct *task, long request,
goto out; goto out;
task_lock(task); task_lock(task);
retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH); retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
task_unlock(task); task_unlock(task);
if (retval) if (retval)
goto unlock_creds; goto unlock_creds;
......
...@@ -299,7 +299,7 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec, ...@@ -299,7 +299,7 @@ static ssize_t process_vm_rw_core(pid_t pid, const struct iovec *lvec,
} }
task_lock(task); task_lock(task);
if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) { if (__ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS)) {
task_unlock(task); task_unlock(task);
rc = -EPERM; rc = -EPERM;
goto put_task_struct; goto put_task_struct;
......
...@@ -141,12 +141,17 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode) ...@@ -141,12 +141,17 @@ int cap_ptrace_access_check(struct task_struct *child, unsigned int mode)
{ {
int ret = 0; int ret = 0;
const struct cred *cred, *child_cred; const struct cred *cred, *child_cred;
const kernel_cap_t *caller_caps;
rcu_read_lock(); rcu_read_lock();
cred = current_cred(); cred = current_cred();
child_cred = __task_cred(child); child_cred = __task_cred(child);
if (mode & PTRACE_MODE_FSCREDS)
caller_caps = &cred->cap_effective;
else
caller_caps = &cred->cap_permitted;
if (cred->user->user_ns == child_cred->user->user_ns && if (cred->user->user_ns == child_cred->user->user_ns &&
cap_issubset(child_cred->cap_permitted, cred->cap_permitted)) cap_issubset(child_cred->cap_permitted, *caller_caps))
goto out; goto out;
if (ns_capable(child_cred->user->user_ns, CAP_SYS_PTRACE)) if (ns_capable(child_cred->user->user_ns, CAP_SYS_PTRACE))
goto out; goto out;
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment