Re: [PATCH] exit: use READ_ONCE() to read mm->owner

[Bradley Morgan]

On 27 October 2026 20:09:23 GMT, Mani Tofigh via B4 Relay
<devnull+mt4022.columbia.edu@xxxxxxxxxx> wrote:
>From: Mani Tofigh <mt4022@xxxxxxxxxxxx>
>
>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@xxxxxxxxxxxxxx>

>
>Signed-off-by: Mani Tofigh <mt4022@xxxxxxxxxxxx>
>---
> 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@xxxxxxxxxxxx>
>
>
>
>From mboxrd@z Thu Jan  1 00:00:00 1970
>From: Mani Tofigh <mt4022@xxxxxxxxxxxx>
>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@xxxxxxxxxx>, 
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, 
> Christian Brauner <brauner@xxxxxxxxxx>
>Cc: linux-mm@xxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, tz2294@xxxxxxxxxxxx, 
> Mani Tofigh <mt4022@xxxxxxxxxxxx>
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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@xxxxxxxxxxxx>
>---
> 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@xxxxxxxxxxxx>
>
>
>


--- Thanks!