Re: [syzbot] [kernel?] WARNING in posixtimer_send_sigqueue

[Hillf Danton <hdanton@sina.com>]

On Thu, 14 Nov 2024 17:49:23 +0100 Frederic Weisbecker <frederic@kernel.org>
Le Wed, Nov 13, 2024 at 04:52:22PM -0800
> > syzbot found the following issue on:
> > 
> > HEAD commit:    929beafbe7ac Add linux-next specific files for 20241108
> > git tree:       linux-next
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=10f714e8580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=75175323f2078363
> > dashboard link: https://syzkaller.appspot.com/bug?extid=852e935b899bde73626e
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14f714e8580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1657b0c0580000
> > 
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/9705ecb6a595/disk-929beafb.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/dbdd1f64b9b8/vmlinux-929beafb.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/3f70d07a929b/bzImage-929beafb.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+852e935b899bde73626e@syzkaller.appspotmail.com
> > 
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 5854 at kernel/signal.c:2008
> > posixtimer_send_sigqueue+0x9da/0xbc0 kernel/signal.c:2008
> 
> That's because prepare_signal() does not only return false when the signal is
> ignored but also when the task group is exiting. It's possible that the task
> enters in do_exit() with pending signal and then the timer is reset and a new
> signal is queued before the sighand dies.
> 
> This should fix it:
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index cbf70c808969..10b464b9d91f 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2003,9 +2003,15 @@ void posixtimer_send_sigqueue(struct k_itimer *tmr)
>  	if (!prepare_signal(sig, t, false)) {
>  		result = TRACE_SIGNAL_IGNORED;
>  
> -		/* Paranoia check. Try to survive. */
> -		if (WARN_ON_ONCE(!list_empty(&q->list)))
> +		if (!list_empty(&q->list)) {
> +			/*
> +			 * If task group is exiting with the signal already pending,
> +			 * wait for __exit_signal() to do its job. Otherwise if
> +			 * ignored, it's not supposed to be queued. Try to survive.
> +			 */
> +			WARN_ON_ONCE(!(t->signal->flags & SIGNAL_GROUP_EXIT));
>  			goto out;
> +		}
>  
>  		/* Periodic timers with SIG_IGN are queued on the ignored list */
>  		if (tmr->it_sig_periodic) {
> 
Test Frederic's patch on top of the next tree.

#syz test

--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2004,9 +2004,15 @@ void posixtimer_send_sigqueue(struct k_i
 	if (!prepare_signal(sig, t, false)) {
 		result = TRACE_SIGNAL_IGNORED;
 
-		/* Paranoia check. Try to survive. */
-		if (WARN_ON_ONCE(!list_empty(&q->list)))
+		if (!list_empty(&q->list)) {
+			/*
+			 * If task group is exiting with the signal already pending,
+			 * wait for __exit_signal() to do its job. Otherwise if
+			 * ignored, it's not supposed to be queued. Try to survive.
+			 */
+			WARN_ON_ONCE(!(t->signal->flags & SIGNAL_GROUP_EXIT));
 			goto out;
+		}
 
 		/* Periodic timers with SIG_IGN are queued on the ignored list */
 		if (tmr->it_sig_periodic) {
--