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signal: Prevent sigqueue caching after task got released

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syzbot reported a memory leak related to sigqueue caching.

The assumption that a task cannot cache a sigqueue after the signal handler
has been dropped and exit_task_sigqueue_cache() has been invoked turns out
to be wrong.

Such a task can still invoke release_task(other_task), which cleans up the
signals of 'other_task' and ends up in sigqueue_cache_or_free(), which in
turn will cache the signal because task->sigqueue_cache is NULL. That's
obviously bogus because nothing will free the cached signal of that task
anymore, so the cached item is leaked.

This happens when e.g. the last non-leader thread exits and reaps the
zombie leader.

Prevent this by setting tsk::sigqueue_cache to an error pointer value in
exit_task_sigqueue_cache() which forces any subsequent invocation of
sigqueue_cache_or_free() from that task to hand the sigqueue back to the
kmemcache.

Add comments to all relevant places.

Fixes: 4bad58ebc8 ("signal: Allow tasks to cache one sigqueue struct")
Reported-by: syzbot+0bac5fec63d4f399ba98@syzkaller.appspotmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/878s32g6j5.ffs@nanos.tec.linutronix.de
This commit is contained in:
Thomas Gleixner 2021-06-22 01:08:30 +02:00
parent 13311e7425
commit 399f8dd9a8
1 changed files with 16 additions and 1 deletions
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17
kernel/signal.c
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@ -435,6 +435,12 @@ __sigqueue_alloc(int sig, struct task_struct *t, gfp_t gfp_flags,
* Preallocation does not hold sighand::siglock so it can't
* use the cache. The lockless caching requires that only
* one consumer and only one producer run at a time.
*
* For the regular allocation case it is sufficient to
* check @q for NULL because this code can only be called
* if the target task @t has not been reaped yet; which
* means this code can never observe the error pointer which is
* written to @t->sigqueue_cache in exit_task_sigqueue_cache().
*/
q = READ_ONCE(t->sigqueue_cache);
if (!q || sigqueue_flags)
@ -463,13 +469,18 @@ void exit_task_sigqueue_cache(struct task_struct *tsk)
struct sigqueue *q = tsk->sigqueue_cache;
if (q) {
tsk->sigqueue_cache = NULL;
/*
* Hand it back to the cache as the task might
* be self reaping which would leak the object.
*/
kmem_cache_free(sigqueue_cachep, q);
}
/*
* Set an error pointer to ensure that @tsk will not cache a
* sigqueue when it is reaping it's child tasks
*/
tsk->sigqueue_cache = ERR_PTR(-1);
}
static void sigqueue_cache_or_free(struct sigqueue *q)
@ -481,6 +492,10 @@ static void sigqueue_cache_or_free(struct sigqueue *q)
* is intentional when run without holding current->sighand->siglock,
* which is fine as current obviously cannot run __sigqueue_free()
* concurrently.
*
* The NULL check is safe even if current has been reaped already,
* in which case exit_task_sigqueue_cache() wrote an error pointer
* into current->sigqueue_cache.
*/
if (!READ_ONCE(current->sigqueue_cache))
WRITE_ONCE(current->sigqueue_cache, q);