Re: [PATCH v2] cgroup/cpuset: Don't access cpuset_cgrp_subsys.root in is_in_v2_mode()

[Andrea Righi <arighi@nvidia.com>]

Hi Waiman,

On Tue, Sep 29, 2026 at 11:18:33PM -0400, 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>

This works for me.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  kernel/cgroup/cpuset.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> [v2] Update the comment as suggested by Ridong
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 19661df6244f..4a2b6f154609 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -152,6 +152,12 @@ static cpumask_var_t	isolated_cpus;		/* CSCB */
>   */
>  static bool		update_housekeeping;	/* RWCS */
>  
> +/*
> + * Set if "cpuset_v2_mode" mount option is used
> + * Cached at bind time and not lock protected; accessed via {READ,WRITE}_ONCE
> + */
> +static bool		cpuset_v2_mode;
> +
>  /*
>   * Copy of isolated_cpus to be passed to housekeeping_update()
>   */
> @@ -439,8 +445,7 @@ 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 +3669,8 @@ 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);
> -- 
> 2.55.0
>