7.2-stable review patch. If anyone has any objections, please let me know. ------------------ From: Waiman Long [ Upstream commit e06b678e3b4e12fdaa0b51a7a5a54abc5dbd6469 ] 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 Reviewed-by: Ridong Chen Reviewed-by: Andrea Righi Signed-off-by: Tejun Heo Signed-off-by: Sasha Levin --- kernel/cgroup/cpuset.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index 2b86b76dba56b..16c554df4a50e 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -151,6 +151,12 @@ 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() */ @@ -403,8 +409,7 @@ 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); } /** @@ -3602,6 +3607,8 @@ 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); -- 2.53.0