[PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Cong Wang]

From: Cong Wang <cwang@multikernel.io>

sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path
even when exec_binprm() fails. For the init task's first execve, this
causes a problem:

1. current->mm is NULL (kernel threads don't have an mm)
2. sched_mm_cid_before_execve() exits early because mm is NULL
3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
4. sched_mm_cid_after_execve() is called with mm still NULL
5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON

This is easily reproduced by booting with an init that is a shell script
(#!/bin/sh) where the interpreter doesn't exist in the initramfs.

Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
matching the behavior of sched_mm_cid_before_execve() which already
handles this case via sched_mm_cid_exit()'s early return.

Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Cong Wang <cwang@multikernel.io>
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 41ba0be16911..60afadb6eede 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10694,10 +10694,11 @@ void sched_mm_cid_before_execve(struct task_struct *t)
 	sched_mm_cid_exit(t);
 }
 
-/* Reactivate MM CID after successful execve() */
+/* Reactivate MM CID after execve() */
 void sched_mm_cid_after_execve(struct task_struct *t)
 {
-	sched_mm_cid_fork(t);
+	if (t->mm)
+		sched_mm_cid_fork(t);
 }
 
 static void mm_cid_work_fn(struct work_struct *work)
-- 
2.34.1

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Mathieu Desnoyers]

On 2025-12-23 16:51, Cong Wang wrote:
> From: Cong Wang <cwang@multikernel.io>
> 
> sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path
> even when exec_binprm() fails. For the init task's first execve, this
> causes a problem:
> 
> 1. current->mm is NULL (kernel threads don't have an mm)
> 2. sched_mm_cid_before_execve() exits early because mm is NULL
> 3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
> 4. sched_mm_cid_after_execve() is called with mm still NULL
> 5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON
> 
> This is easily reproduced by booting with an init that is a shell script
> (#!/bin/sh) where the interpreter doesn't exist in the initramfs.
> 
> Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
> matching the behavior of sched_mm_cid_before_execve() which already
> handles this case via sched_mm_cid_exit()'s early return.
 >
 > Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
 > Cc: Thomas Gleixner <tglx@linutronix.de>
 > Signed-off-by: Cong Wang <cwang@multikernel.io>

Thanks for the detailed explanation.

Indeed, the offending commit removes a pre-existing NULL mm check:

  void sched_mm_cid_after_execve(struct task_struct *t)
  {
-       struct mm_struct *mm = t->mm;
-
-       if (!mm)
-               return;

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Thomas, Peter, Ingo, can we fast-track this fix for upstream ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Qing Wang]

>  void sched_mm_cid_after_execve(struct task_struct *t)
>  {
> -    sched_mm_cid_fork(t);
> +    if (t->mm)
> +        sched_mm_cid_fork(t);
>  }

Hi,

It's a correct solution, but I have a small suggestion that putting the 'mm'
checking into sched_mm_cid_fork(), just like sched_mm_cid_exit().

Best regards,
Qing Wang

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Will Deacon]

On Tue, Dec 23, 2025 at 01:51:13PM -0800, Cong Wang wrote:
> From: Cong Wang <cwang@multikernel.io>
> 
> sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path
> even when exec_binprm() fails. For the init task's first execve, this
> causes a problem:
> 
> 1. current->mm is NULL (kernel threads don't have an mm)
> 2. sched_mm_cid_before_execve() exits early because mm is NULL
> 3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
> 4. sched_mm_cid_after_execve() is called with mm still NULL
> 5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON
> 
> This is easily reproduced by booting with an init that is a shell script
> (#!/bin/sh) where the interpreter doesn't exist in the initramfs.
> 
> Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
> matching the behavior of sched_mm_cid_before_execve() which already
> handles this case via sched_mm_cid_exit()'s early return.
> 
> Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Cong Wang <cwang@multikernel.io>
> ---
>  kernel/sched/core.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 41ba0be16911..60afadb6eede 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -10694,10 +10694,11 @@ void sched_mm_cid_before_execve(struct task_struct *t)
>  	sched_mm_cid_exit(t);
>  }
>  
> -/* Reactivate MM CID after successful execve() */
> +/* Reactivate MM CID after execve() */
>  void sched_mm_cid_after_execve(struct task_struct *t)
>  {
> -	sched_mm_cid_fork(t);
> +	if (t->mm)
> +		sched_mm_cid_fork(t);
>  }
>  
>  static void mm_cid_work_fn(struct work_struct *work)

This addresses a panic reported on arm64 when trying to execute x86
binaries using TCG on an Apple device:

https://lore.kernel.org/all/20251226192506.88593-1-za4emsu@gmail.com/

so:

Acked-by: Will Deacon <will@kernel.org>

Please can we land this for 6.19?

Cheers,

Will

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Mathieu Desnoyers]

On 2026-01-07 13:00, Will Deacon wrote:
> On Tue, Dec 23, 2025 at 01:51:13PM -0800, Cong Wang wrote:
>> From: Cong Wang <cwang@multikernel.io>
>>
>> sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path
>> even when exec_binprm() fails. For the init task's first execve, this
>> causes a problem:
>>
>> 1. current->mm is NULL (kernel threads don't have an mm)
>> 2. sched_mm_cid_before_execve() exits early because mm is NULL
>> 3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
>> 4. sched_mm_cid_after_execve() is called with mm still NULL
>> 5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON
>>
>> This is easily reproduced by booting with an init that is a shell script
>> (#!/bin/sh) where the interpreter doesn't exist in the initramfs.
>>
>> Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
>> matching the behavior of sched_mm_cid_before_execve() which already
>> handles this case via sched_mm_cid_exit()'s early return.
>>
>> Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Signed-off-by: Cong Wang <cwang@multikernel.io>
>> ---
>>   kernel/sched/core.c | 5 +++--
>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 41ba0be16911..60afadb6eede 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -10694,10 +10694,11 @@ void sched_mm_cid_before_execve(struct task_struct *t)
>>   	sched_mm_cid_exit(t);
>>   }
>>   
>> -/* Reactivate MM CID after successful execve() */
>> +/* Reactivate MM CID after execve() */
>>   void sched_mm_cid_after_execve(struct task_struct *t)
>>   {
>> -	sched_mm_cid_fork(t);
>> +	if (t->mm)
>> +		sched_mm_cid_fork(t);
>>   }
>>   
>>   static void mm_cid_work_fn(struct work_struct *work)
> 
> This addresses a panic reported on arm64 when trying to execute x86
> binaries using TCG on an Apple device:
> 
> https://lore.kernel.org/all/20251226192506.88593-1-za4emsu@gmail.com/
> 
> so:
> 
> Acked-by: Will Deacon <will@kernel.org>
> 
> Please can we land this for 6.19?

Yes, please. I gave my Reviewed-by already 2 weeks ago:

https://lore.kernel.org/lkml/d6ac8fe9-5fc0-4042-9592-cde3db82b65e@efficios.com/

I guess the relevant maintainers are gradually coming back from the holiday break.
I will ask it again here: can we fast-track this fix for upstream ?

Thanks,

Mathieu

> 
> Cheers,
> 
> Will


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Thomas Gleixner]

On Thu, Jan 08 2026 at 10:28, Mathieu Desnoyers wrote:
> On 2026-01-07 13:00, Will Deacon wrote:
> I guess the relevant maintainers are gradually coming back from the holiday break.
> I will ask it again here: can we fast-track this fix for upstream ?

Yes people are coming back from vacation and are picking up stuff.

[tip: sched/urgent] sched/mm_cid: Prevent NULL mm dereference in sched_mm_cid_after_execve()

[tip-bot2 for Cong Wang]

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     2bdf777410dc6e022d1081885ff34673b5dfee99
Gitweb:        https://git.kernel.org/tip/2bdf777410dc6e022d1081885ff34673b5dfee99
Author:        Cong Wang <cwang@multikernel.io>
AuthorDate:    Tue, 23 Dec 2025 13:51:13 -08:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Fri, 09 Jan 2026 13:02:57 +01:00

sched/mm_cid: Prevent NULL mm dereference in sched_mm_cid_after_execve()

sched_mm_cid_after_execve() is called in bprm_execve()'s cleanup path even
when exec_binprm() fails. For the init task's first execve(), this causes a
problem:

  1. current->mm is NULL (kernel threads don't have an mm)
  2. sched_mm_cid_before_execve() exits early because mm is NULL
  3. exec_binprm() fails (e.g., ENOENT for missing script interpreter)
  4. sched_mm_cid_after_execve() is called with mm still NULL
  5. sched_mm_cid_fork() is called unconditionally, triggering WARN_ON

This is easily reproduced by booting with an init that is a shell script
(#!/bin/sh) where the interpreter doesn't exist in the initramfs.

Fix this by checking if t->mm is NULL before calling sched_mm_cid_fork(),
matching the behavior of sched_mm_cid_before_execve() which already
handles this case via sched_mm_cid_exit()'s early return.

Fixes: b0c3d51b54f8 ("sched/mmcid: Provide precomputed maximal value")
Signed-off-by: Cong Wang <cwang@multikernel.io>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20251223215113.639686-1-xiyou.wangcong@gmail.com
---
 kernel/sched/core.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 41ba0be..60afadb 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10694,10 +10694,11 @@ void sched_mm_cid_before_execve(struct task_struct *t)
 	sched_mm_cid_exit(t);
 }
 
-/* Reactivate MM CID after successful execve() */
+/* Reactivate MM CID after execve() */
 void sched_mm_cid_after_execve(struct task_struct *t)
 {
-	sched_mm_cid_fork(t);
+	if (t->mm)
+		sched_mm_cid_fork(t);
 }
 
 static void mm_cid_work_fn(struct work_struct *work)

Re: [PATCH] sched: Fix NULL mm dereference in sched_mm_cid_after_execve()

[Mathieu Desnoyers]

On 2026-01-09 06:53, Thomas Gleixner wrote:
> On Thu, Jan 08 2026 at 10:28, Mathieu Desnoyers wrote:
>> On 2026-01-07 13:00, Will Deacon wrote:
>> I guess the relevant maintainers are gradually coming back from the holiday break.
>> I will ask it again here: can we fast-track this fix for upstream ?
> 
> Yes people are coming back from vacation and are picking up stuff.

That's what I figured. Thanks for picking this up!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com