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

[Waiman Long]

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@xxxxxxxxxx

Fixes: b8d1b8ee93df ("cpuset: Allow v2 behavior in v1 cgroup")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 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