Re: [PATCH] cgroup/cpuset: Don't access cpuset_cgrp_subsys.root in is_in_v2_mode()

[Ridong Chen <ridong.chen@linux.dev>]

On 9/30/2026 9:28 AM, Waiman Long wrote:
> After seeing the patch [1] to guard is_in_v2_mode() with RCU, it makes
> me realize that is_in_v2_mode() may be called in a context where a new
> cgroup filesystem is being rebound with stale cpuset_cgrp_subsys.root
> pointer. Avoid this potential UaF situation by adding a new cpuset_v2_mode
> flag which is set when the cpuset_v2_mode mount option is used. This
> flag is written into only when cpuset_bind() is being called with a
> stable cpuset_cgrp_subsys.root value. The is_in_v2_mode() helper is
> modified to read the new cpuset_v2_mode flag instead of accessing
> cpuset_cgrp_subsys.root directly.
> 
> [1] https://lore.kernel.org/lkml/20260929084124.626693-2-arighi@nvidia.com
> 
> Fixes: b8d1b8ee93df ("cpuset: Allow v2 behavior in v1 cgroup")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 10 ++++++++--
>   1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 19661df6244f..266ce17d1af0 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -147,6 +147,11 @@ static cpumask_var_t	subpartitions_cpus;	/* RWCS */
>    */
>   static cpumask_var_t	isolated_cpus;		/* CSCB */
>   
> +/*
> + * Set if "cpuset_v2_mode" mount option is used
> + */
> +static bool		cpuset_v2_mode;		/* Unprotected */
> +

Nit.

`Unprotected` is wired here, will it be better with:

/* Cached at bind time; accessed via {READ,WRITE}_ONCE */

>   /*
>    * Set if housekeeping cpumasks are to be updated.
>    */
> @@ -439,8 +444,7 @@ static inline bool cpuset_v2(void)
>    */
>   static inline bool is_in_v2_mode(void)
>   {
> -	return cpuset_v2() ||
> -	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
> +	return cpuset_v2() || READ_ONCE(cpuset_v2_mode);
>   }
>   
>   /**
> @@ -3664,6 +3668,8 @@ static void cpuset_bind(struct cgroup_subsys_state *root_css)
>   	mutex_lock(&cpuset_mutex);
>   	spin_lock_irq(&callback_lock);
>   
> +	WRITE_ONCE(cpuset_v2_mode,
> +		   !!(cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE));
>   	if (is_in_v2_mode()) {
>   		cpumask_copy(top_cpuset.cpus_allowed, cpu_possible_mask);
>   		cpumask_copy(top_cpuset.effective_xcpus, cpu_possible_mask);


Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Thanks.

-- 
Best regards
Ridong