Commit 51891498 authored by Tycho Andersen's avatar Tycho Andersen Committed by Kees Cook

seccomp: allow TSYNC and USER_NOTIF together

The restriction introduced in 7a0df7fb ("seccomp: Make NEW_LISTENER and
TSYNC flags exclusive") is mostly artificial: there is enough information
in a seccomp user notification to tell which thread triggered a
notification. The reason it was introduced is because TSYNC makes the
syscall return a thread-id on failure, and NEW_LISTENER returns an fd, and
there's no way to distinguish between these two cases (well, I suppose the
caller could check all fds it has, then do the syscall, and if the return
value was an fd that already existed, then it must be a thread id, but
bleh).

Matthew would like to use these two flags together in the Chrome sandbox
which wants to use TSYNC for video drivers and NEW_LISTENER to proxy
syscalls.

So, let's fix this ugliness by adding another flag, TSYNC_ESRCH, which
tells the kernel to just return -ESRCH on a TSYNC error. This way,
NEW_LISTENER (and any subsequent seccomp() commands that want to return
positive values) don't conflict with each other.
Suggested-by: default avatarMatthew Denton <mpdenton@google.com>
Signed-off-by: default avatarTycho Andersen <tycho@tycho.ws>
Link: https://lore.kernel.org/r/20200304180517.23867-1-tycho@tycho.wsSigned-off-by: default avatarKees Cook <keescook@chromium.org>
parent 11a48a5a
......@@ -7,7 +7,8 @@
#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC | \
SECCOMP_FILTER_FLAG_LOG | \
SECCOMP_FILTER_FLAG_SPEC_ALLOW | \
SECCOMP_FILTER_FLAG_NEW_LISTENER)
SECCOMP_FILTER_FLAG_NEW_LISTENER | \
SECCOMP_FILTER_FLAG_TSYNC_ESRCH)
#ifdef CONFIG_SECCOMP
......
......@@ -22,6 +22,7 @@
#define SECCOMP_FILTER_FLAG_LOG (1UL << 1)
#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
#define SECCOMP_FILTER_FLAG_NEW_LISTENER (1UL << 3)
#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4)
/*
* All BPF programs must return a 32-bit value.
......
......@@ -528,8 +528,12 @@ static long seccomp_attach_filter(unsigned int flags,
int ret;
ret = seccomp_can_sync_threads();
if (ret)
return ret;
if (ret) {
if (flags & SECCOMP_FILTER_FLAG_TSYNC_ESRCH)
return -ESRCH;
else
return ret;
}
}
/* Set log flag, if present. */
......@@ -1288,10 +1292,12 @@ static long seccomp_set_mode_filter(unsigned int flags,
* In the successful case, NEW_LISTENER returns the new listener fd.
* But in the failure case, TSYNC returns the thread that died. If you
* combine these two flags, there's no way to tell whether something
* succeeded or failed. So, let's disallow this combination.
* succeeded or failed. So, let's disallow this combination if the user
* has not explicitly requested no errors from TSYNC.
*/
if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
(flags & SECCOMP_FILTER_FLAG_NEW_LISTENER))
(flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) &&
((flags & SECCOMP_FILTER_FLAG_TSYNC_ESRCH) == 0))
return -EINVAL;
/* Prepare the new filter before holding any locks. */
......
......@@ -212,6 +212,10 @@ struct seccomp_notif_sizes {
#define SECCOMP_USER_NOTIF_FLAG_CONTINUE 0x00000001
#endif
#ifndef SECCOMP_FILTER_FLAG_TSYNC_ESRCH
#define SECCOMP_FILTER_FLAG_TSYNC_ESRCH (1UL << 4)
#endif
#ifndef seccomp
int seccomp(unsigned int op, unsigned int flags, void *args)
{
......@@ -2187,7 +2191,8 @@ TEST(detect_seccomp_filter_flags)
unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
SECCOMP_FILTER_FLAG_LOG,
SECCOMP_FILTER_FLAG_SPEC_ALLOW,
SECCOMP_FILTER_FLAG_NEW_LISTENER };
SECCOMP_FILTER_FLAG_NEW_LISTENER,
SECCOMP_FILTER_FLAG_TSYNC_ESRCH };
unsigned int exclusive[] = {
SECCOMP_FILTER_FLAG_TSYNC,
SECCOMP_FILTER_FLAG_NEW_LISTENER };
......@@ -2645,6 +2650,55 @@ TEST_F(TSYNC, two_siblings_with_one_divergence)
EXPECT_EQ(SIBLING_EXIT_UNKILLED, (long)status);
}
TEST_F(TSYNC, two_siblings_with_one_divergence_no_tid_in_err)
{
long ret, flags;
void *status;
ASSERT_EQ(0, prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
}
ret = seccomp(SECCOMP_SET_MODE_FILTER, 0, &self->root_prog);
ASSERT_NE(ENOSYS, errno) {
TH_LOG("Kernel does not support seccomp syscall!");
}
ASSERT_EQ(0, ret) {
TH_LOG("Kernel does not support SECCOMP_SET_MODE_FILTER!");
}
self->sibling[0].diverge = 1;
tsync_start_sibling(&self->sibling[0]);
tsync_start_sibling(&self->sibling[1]);
while (self->sibling_count < TSYNC_SIBLINGS) {
sem_wait(&self->started);
self->sibling_count++;
}
flags = SECCOMP_FILTER_FLAG_TSYNC | \
SECCOMP_FILTER_FLAG_TSYNC_ESRCH;
ret = seccomp(SECCOMP_SET_MODE_FILTER, flags, &self->apply_prog);
ASSERT_EQ(ESRCH, errno) {
TH_LOG("Did not return ESRCH for diverged sibling.");
}
ASSERT_EQ(-1, ret) {
TH_LOG("Did not fail on diverged sibling.");
}
/* Wake the threads */
pthread_mutex_lock(&self->mutex);
ASSERT_EQ(0, pthread_cond_broadcast(&self->cond)) {
TH_LOG("cond broadcast non-zero");
}
pthread_mutex_unlock(&self->mutex);
/* Ensure they are both unkilled. */
PTHREAD_JOIN(self->sibling[0].tid, &status);
EXPECT_EQ(SIBLING_EXIT_UNKILLED, (long)status);
PTHREAD_JOIN(self->sibling[1].tid, &status);
EXPECT_EQ(SIBLING_EXIT_UNKILLED, (long)status);
}
TEST_F(TSYNC, two_siblings_not_under_filter)
{
long ret, sib;
......@@ -3196,6 +3250,24 @@ TEST(user_notification_basic)
EXPECT_EQ(0, WEXITSTATUS(status));
}
TEST(user_notification_with_tsync)
{
int ret;
unsigned int flags;
/* these were exclusive */
flags = SECCOMP_FILTER_FLAG_NEW_LISTENER |
SECCOMP_FILTER_FLAG_TSYNC;
ASSERT_EQ(-1, user_trap_syscall(__NR_getppid, flags));
ASSERT_EQ(EINVAL, errno);
/* but now they're not */
flags |= SECCOMP_FILTER_FLAG_TSYNC_ESRCH;
ret = user_trap_syscall(__NR_getppid, flags);
close(ret);
ASSERT_LE(0, ret);
}
TEST(user_notification_kill_in_middle)
{
pid_t pid;
......
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