* 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 23:11 ` Bradley Morgan
2026-10-27 23:16 ` Balbir Singh
2026-10-28 14:37 ` Oleg Nesterov
2 siblings, 0 replies; 4+ messages in thread
From: Bradley Morgan @ 2026-10-27 23:11 UTC (permalink / raw)
To: devnull+mt4022.columbia.edu
Cc: akpm, brauner, linux-kernel, linux-mm, mt4022, oleg, tz2294
On 27 October 2026 20:09:23 GMT, Mani Tofigh via B4 Relay
<devnull+mt4022.columbia.edu@kernel.org> 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.
Good fix, not complaining
Reviewed-by: Bradley Morgan <brads@mainlining.org>
>
>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)
Wow!
> 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>
>
>
>
>From mboxrd@z Thu Jan 1 00:00:00 1970
>From: Mani Tofigh <mt4022@columbia.edu>
>Date: Tue, 27 Oct 2026 16:09:23 -0400
>Subject: [PATCH] exit: use READ_ONCE() to read mm->owner
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>To: Oleg Nesterov <oleg@redhat.com>,
> Andrew Morton <akpm@linux-foundation.org>,
> Christian Brauner <brauner@kernel.org>
>Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, tz2294@columbia.edu,
> Mani Tofigh <mt4022@columbia.edu>
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>
>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>
>
>
>
--- Thanks!
^ permalink raw reply [flat|nested] 4+ 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 23:11 ` Bradley Morgan
@ 2026-10-27 23:16 ` Balbir Singh
2026-10-28 14:37 ` Oleg Nesterov
2 siblings, 0 replies; 4+ 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] 4+ messages in thread