* [PATCH] exit: use READ_ONCE() to read mm->owner
@ 2026-10-27 20:09 Mani Tofigh via B4 Relay
2026-10-27 23:16 ` Balbir Singh
` (2 more replies)
0 siblings, 3 replies; 6+ messages in thread
From: Mani Tofigh via B4 Relay @ 2026-10-27 20:09 UTC (permalink / raw)
To: Oleg Nesterov, Andrew Morton, Christian Brauner
Cc: linux-mm, linux-kernel, tz2294, Mani Tofigh
From: Mani Tofigh <mt4022@columbia.edu>
mm->owner is written with WRITE_ONCE() when ownership moves to another
task, but mm_update_next_owner() reads it with a plain load. The race
is expected, so read it with READ_ONCE() to match the writer.
[ 14.915975] BUG: KCSAN: data-race in mm_update_next_owner / try_to_set_owner
[ 14.916154] write (marked) to 0xff11000004471b80 of 8 bytes by task 71 on cpu 0:
[ 14.916204] try_to_set_owner+0x100/0x140
[ 14.916302] mm_update_next_owner+0x177/0x300
...
[ 14.917246] read to 0xff11000004471b80 of 8 bytes by task 75 on cpu 1:
[ 14.917293] mm_update_next_owner+0x2a/0x300
[ 14.917396] do_exit+0x575/0x18b0
...
[ 14.923906] value changed: 0xff1100002ec04180 -> 0xff1100002ed3a0c0
Found with KCSAN while fuzzing under a deterministic hypervisor that's
currently under development.
Signed-off-by: Mani Tofigh <mt4022@columbia.edu>
---
kernel/exit.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/exit.c b/kernel/exit.c
index d5e7bc2af804..cc78596db74d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -494,7 +494,7 @@ void mm_update_next_owner(struct mm_struct *mm)
* If the exiting or execing task is not the owner, it's
* someone else's problem.
*/
- if (mm->owner != p)
+ if (READ_ONCE(mm->owner) != p)
return;
/*
* The current owner is exiting/execing and there are no other
---
base-commit: d135ab453a7400a34930bdda1ef4bdc8acfac80e
change-id: 20261027-kcsan-exit-109dce625d81
Best regards,
--
Mani Tofigh <mt4022@columbia.edu>
^ permalink raw reply related [flat|nested] 6+ messages in thread* Re: [PATCH] exit: use READ_ONCE() to read mm->owner
2026-10-27 20:09 [PATCH] exit: use READ_ONCE() to read mm->owner Mani Tofigh via B4 Relay
@ 2026-10-27 23:16 ` Balbir Singh
2026-10-28 14:37 ` Oleg Nesterov
2026-10-29 6:54 ` Andrew Morton
2 siblings, 0 replies; 6+ messages in thread
From: Balbir Singh @ 2026-10-27 23:16 UTC (permalink / raw)
To: mt4022
Cc: Oleg Nesterov, Andrew Morton, Christian Brauner, linux-mm,
linux-kernel, tz2294
On Tue, Oct 27, 2026 at 04:09:23PM -0400, Mani Tofigh via B4 Relay wrote:
> From: Mani Tofigh <mt4022@columbia.edu>
>
> mm->owner is written with WRITE_ONCE() when ownership moves to another
> task, but mm_update_next_owner() reads it with a plain load. The race
> is expected, so read it with READ_ONCE() to match the writer.
>
> [ 14.915975] BUG: KCSAN: data-race in mm_update_next_owner / try_to_set_owner
>
> [ 14.916154] write (marked) to 0xff11000004471b80 of 8 bytes by task 71 on cpu 0:
> [ 14.916204] try_to_set_owner+0x100/0x140
> [ 14.916302] mm_update_next_owner+0x177/0x300
> ...
>
> [ 14.917246] read to 0xff11000004471b80 of 8 bytes by task 75 on cpu 1:
> [ 14.917293] mm_update_next_owner+0x2a/0x300
> [ 14.917396] do_exit+0x575/0x18b0
> ...
>
> [ 14.923906] value changed: 0xff1100002ec04180 -> 0xff1100002ed3a0c0
>
> Found with KCSAN while fuzzing under a deterministic hypervisor that's
> currently under development.
>
> Signed-off-by: Mani Tofigh <mt4022@columbia.edu>
> ---
> kernel/exit.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index d5e7bc2af804..cc78596db74d 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -494,7 +494,7 @@ void mm_update_next_owner(struct mm_struct *mm)
> * If the exiting or execing task is not the owner, it's
> * someone else's problem.
> */
> - if (mm->owner != p)
> + if (READ_ONCE(mm->owner) != p)
> return;
> /*
> * The current owner is exiting/execing and there are no other
>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Balbir
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] exit: use READ_ONCE() to read mm->owner
2026-10-27 20:09 [PATCH] exit: use READ_ONCE() to read mm->owner Mani Tofigh via B4 Relay
2026-10-27 23:16 ` Balbir Singh
@ 2026-10-28 14:37 ` Oleg Nesterov
2026-10-29 6:54 ` Andrew Morton
2 siblings, 0 replies; 6+ messages in thread
From: Oleg Nesterov @ 2026-10-28 14:37 UTC (permalink / raw)
To: Mani Tofigh via B4 Relay
Cc: Andrew Morton, Christian Brauner, linux-mm, linux-kernel, tz2294,
Mani Tofigh
On 10/27, Mani Tofigh via B4 Relay wrote:
>
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -494,7 +494,7 @@ void mm_update_next_owner(struct mm_struct *mm)
> * If the exiting or execing task is not the owner, it's
> * someone else's problem.
> */
> - if (mm->owner != p)
> + if (READ_ONCE(mm->owner) != p)
Acked-by: Oleg Nesterov <oleg@redhat.com>
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [PATCH] exit: use READ_ONCE() to read mm->owner
2026-10-27 20:09 [PATCH] exit: use READ_ONCE() to read mm->owner Mani Tofigh via B4 Relay
2026-10-27 23:16 ` Balbir Singh
2026-10-28 14:37 ` Oleg Nesterov
@ 2026-10-29 6:54 ` Andrew Morton
2026-10-29 15:05 ` Mani Tofigh
2 siblings, 1 reply; 6+ messages in thread
From: Andrew Morton @ 2026-10-29 6:54 UTC (permalink / raw)
To: mt4022
Cc: Mani Tofigh via B4 Relay, Oleg Nesterov, Christian Brauner,
linux-mm, linux-kernel, tz2294
On Tue, 27 Oct 2026 16:09:23 -0400 Mani Tofigh via B4 Relay <devnull+mt4022.columbia.edu@kernel.org> wrote:
> mm->owner is written with WRITE_ONCE() when ownership moves to another
> task, but mm_update_next_owner() reads it with a plain load. The race
> is expected, so read it with READ_ONCE() to match the writer.
>
> [ 14.915975] BUG: KCSAN: data-race in mm_update_next_owner / try_to_set_owner
>
> [ 14.916154] write (marked) to 0xff11000004471b80 of 8 bytes by task 71 on cpu 0:
> [ 14.916204] try_to_set_owner+0x100/0x140
> [ 14.916302] mm_update_next_owner+0x177/0x300
> ...
>
> [ 14.917246] read to 0xff11000004471b80 of 8 bytes by task 75 on cpu 1:
> [ 14.917293] mm_update_next_owner+0x2a/0x300
> [ 14.917396] do_exit+0x575/0x18b0
> ...
>
> [ 14.923906] value changed: 0xff1100002ec04180 -> 0xff1100002ed3a0c0
>
> Found with KCSAN while fuzzing under a deterministic hypervisor that's
> currently under development.
We don't want older kernels doing this either, so I suggest
Fixes: cf475ad28ac3 ("cgroups: add an owner to the mm_struct")
Cc: <stable$vger.kernel.org>
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] exit: use READ_ONCE() to read mm->owner
2026-10-29 6:54 ` Andrew Morton
@ 2026-10-29 15:05 ` Mani Tofigh
2026-10-29 16:38 ` Andrew Morton
0 siblings, 1 reply; 6+ messages in thread
From: Mani Tofigh @ 2026-10-29 15:05 UTC (permalink / raw)
To: Andrew Morton
Cc: Oleg Nesterov, Christian Brauner, linux-mm, linux-kernel, tz2294
On 10/29/26 2:54 AM, Andrew Morton wrote:
> We don't want older kernels doing this either, so I suggest
>
> Fixes: cf475ad28ac3 ("cgroups: add an owner to the mm_struct")
> Cc: <stable$vger.kernel.org>
Sounds good to me, thanks. Should I send a v2 with these tags or will
you add them when you apply it?
Mani
^ permalink raw reply [flat|nested] 6+ messages in thread* Re: [PATCH] exit: use READ_ONCE() to read mm->owner
2026-10-29 15:05 ` Mani Tofigh
@ 2026-10-29 16:38 ` Andrew Morton
0 siblings, 0 replies; 6+ messages in thread
From: Andrew Morton @ 2026-10-29 16:38 UTC (permalink / raw)
To: Mani Tofigh
Cc: Oleg Nesterov, Christian Brauner, linux-mm, linux-kernel, tz2294
On Thu, 29 Oct 2026 11:05:02 -0400 Mani Tofigh <mt4022@columbia.edu> wrote:
>
>
> On 10/29/26 2:54 AM, Andrew Morton wrote:
> > We don't want older kernels doing this either, so I suggest
> >
> > Fixes: cf475ad28ac3 ("cgroups: add an owner to the mm_struct")
> > Cc: <stable$vger.kernel.org>
>
> Sounds good to me, thanks. Should I send a v2 with these tags or will
> you add them when you apply it?
>
That's OK - we gather up the patch metadata as a part of the merging
process.
I'm assuming that Christian will be handling this patch.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2026-10-29 16:38 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2026-10-27 20:09 [PATCH] exit: use READ_ONCE() to read mm->owner Mani Tofigh via B4 Relay
2026-10-27 23:16 ` Balbir Singh
2026-10-28 14:37 ` Oleg Nesterov
2026-10-29 6:54 ` Andrew Morton
2026-10-29 15:05 ` Mani Tofigh
2026-10-29 16:38 ` Andrew Morton
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox