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

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

[Mani Tofigh via B4 Relay]

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>

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@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
>MIME-Version: 1.0
>Content-Type: text/plain; charset="utf-8"
>Content-Transfer-Encoding: 7bit
>Message-Id: <20261027-kcsan-exit-v1-1-7c0429a29bc0@columbia.edu>
>X-B4-Tracking: v=1; b=H4sIAAAAAAAC/yXMTQqAIBBA4avErBNU6Pcq0aJ0qimwcCyC6O5ZL
> b/FexcwekKGOrnA40FMq4tQaQJm6tyIgmw0aKlzJXUhFsOdE3hSEEpW1mCuM1sqiMHmcaDzmzX
> tb977GU14D3DfD5lpoW9uAAAA
>X-Change-ID: 20261027-kcsan-exit-109dce625d81
>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>
>X-Mailer: b4 0.16.0
>X-Developer-Signature: v=1; a=ed25519-sha256; t=1793131763; l=1574;
> i=mt4022@columbia.edu; s=20261026; h=from:subject:message-id;
> bh=WXjCMdCN2l86r1jkTnC4XOf4kZ6T9fA2oOWN9xZ8maQ=;
> b=P4fzFN8wMHtZOrSvTz2cGZqT83oLhWqh1Yl+Q6idpSUy4YBkJ/VD293cGAp/MVmQ0NcTSTKL6
> UTAzDy6KdpKAIEtlMq8vulmMt2Se0lha0gFkV8W1YvL068x2hwvXryx
>X-Developer-Key: i=mt4022@columbia.edu; a=ed25519;
> pk=gljCMQDuxyKPmd5y37CRpEJGSPXDV1MyAnHBhCgXl0I=
>X-Endpoint-Received: by B4 Relay for mt4022@columbia.edu/20261026 with auth_id=1322
>List-Id: B4 Relay Submissions <b4-sent.feeds.kernel.org>
>
>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!

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

[Balbir Singh]

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