signal: Prevent sigqueue caching after task got released

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: 4bad58ebc8bc ("signal: Allow tasks to cache one sigqueue struct")
Reported-by: [email protected]
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Oleg Nesterov <[email protected]>
Acked-by: Christian Brauner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

0 files changed, 0 insertions, 0 deletions