Commit 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in
cgroup") separates out the freeing of tmpmasks into a new free_tmpmask()
helper but removes the NULL pointer check in the process. Unfortunately a
NULL pointer can be passed to free_tmpmasks() in cpuset_handle_hotplug()
if cpuset v1 is active. This can cause segmentation fault and crash
the kernel.

Fix that by adding the NULL pointer check to free_tmpmasks().

Fixes: 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in cgroup")
Reported-by: Ashay Jaiswal <quic_ashayj@quicinc.com>
Closes: https://lore.kernel.org/lkml/20250902-cpuset-free-on-condition-v1-1-f46ffab53eac@quicinc.com/
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a78ccd11ce9b..c0c281a8860d 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -484,6 +484,9 @@ static inline int alloc_tmpmasks(struct tmpmasks *tmp)
  */
 static inline void free_tmpmasks(struct tmpmasks *tmp)
 {
+	if (!tmp)
+		return;
+
 	free_cpumask_var(tmp->new_cpus);
 	free_cpumask_var(tmp->addmask);
 	free_cpumask_var(tmp->delmask);
-- 
2.50.1

On Tue, Sep 02, 2025 at 02:15:37PM -0400, Waiman Long wrote:
> Commit 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in
> cgroup") separates out the freeing of tmpmasks into a new free_tmpmask()
> helper but removes the NULL pointer check in the process. Unfortunately a
> NULL pointer can be passed to free_tmpmasks() in cpuset_handle_hotplug()
> if cpuset v1 is active. This can cause segmentation fault and crash
> the kernel.
> 
> Fix that by adding the NULL pointer check to free_tmpmasks().
> 
> Fixes: 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in cgroup")
> Reported-by: Ashay Jaiswal <quic_ashayj@quicinc.com>
> Closes: https://lore.kernel.org/lkml/20250902-cpuset-free-on-condition-v1-1-f46ffab53eac@quicinc.com/
> Signed-off-by: Waiman Long <longman@redhat.com>

Applied to cgroup/for-6.18.

Thanks.

-- 
tejun

On 2025/9/3 2:15, Waiman Long wrote:
> Commit 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in
> cgroup") separates out the freeing of tmpmasks into a new free_tmpmask()
> helper but removes the NULL pointer check in the process. Unfortunately a
> NULL pointer can be passed to free_tmpmasks() in cpuset_handle_hotplug()
> if cpuset v1 is active. This can cause segmentation fault and crash
> the kernel.
> 
> Fix that by adding the NULL pointer check to free_tmpmasks().
> 
> Fixes: 5806b3d05165 ("cpuset: decouple tmpmasks and cpumasks freeing in cgroup")
> Reported-by: Ashay Jaiswal <quic_ashayj@quicinc.com>
> Closes: https://lore.kernel.org/lkml/20250902-cpuset-free-on-condition-v1-1-f46ffab53eac@quicinc.com/
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a78ccd11ce9b..c0c281a8860d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -484,6 +484,9 @@ static inline int alloc_tmpmasks(struct tmpmasks *tmp)
>   */
>  static inline void free_tmpmasks(struct tmpmasks *tmp)
>  {
> +	if (!tmp)
> +		return;
> +
>  	free_cpumask_var(tmp->new_cpus);
>  	free_cpumask_var(tmp->addmask);
>  	free_cpumask_var(tmp->delmask);

Thank you, Longman and Ashay.

My apologies for missing the NULL pointer check, which led to this issue.
I have double-checked the free_cpuset function, and it does not receive a NULL pointer as input.

-- 
Best regards,
Ridong