signals: fix sigqueue_free() vs __exit_signal() race

__exit_signal() does flush_sigqueue(tsk->pending) outside of ->siglock.
This can race with another thread doing sigqueue_free(), we can free the
same SIGQUEUE_PREALLOC sigqueue twice or corrupt the pending->list.

Note that even sys_exit_group() can trigger this race, not only
sys_timer_delete().

Move the callsite of flush_sigqueue(tsk->pending) under ->siglock.

This patch doesn't touch flush_sigqueue(->shared_pending) below, it is
called when there are no other threads which can play with signals, and
sigqueue_free() can't be used outside of our thread group.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Acked-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/signal.c b/kernel/signal.c
index 72bb4f51f963..12ffea7c201d 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1242,7 +1242,8 @@ void sigqueue_free(struct sigqueue *q)
         /*
          * If the signal is still pending remove it from the
          * pending queue. We must hold ->siglock while testing
-         * q->list to serialize with collect_signal().
+         * q->list to serialize with collect_signal() or with
+         * __exit_signal()->flush_sigqueue().
          */
         spin_lock_irqsave(lock, flags);
         if (!list_empty(&q->list))