[PATCH] nsfs: keep namespace tree fields stable until after RCU grace period

struct ns_common overlays struct ns_tree with the rcu_head used by kfree_rcu(). 
That lets the deferred-free machinery overwrite ns_id and __ns_ref_active as
soon as a namespace is queued for freeing, even though nsfs tree walkers may
still read those fields under RCU after ns_tree_remove(). KASAN reports slab
UAF.

Keep the tree state and deferred-free callback storage separate. Namespace tree
readers can then continue to validate and take references until the grace
period has elapsed.

Assisted-by: Codex:gpt-5
Signed-off-by: Jérémy Jean <[email protected]>
---
 include/linux/ns/ns_common_types.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/ns/ns_common_types.h b/include/linux/ns/ns_common_types.h
index ea45c54e4435..6ed6b497831c 100644
--- a/include/linux/ns/ns_common_types.h
+++ b/include/linux/ns/ns_common_types.h
@@ -116,10 +116,8 @@ struct ns_common {
 	struct dentry *stashed;
 	const struct proc_ns_operations *ops;
 	unsigned int inum;
-	union {
-		struct ns_tree;
-		struct rcu_head ns_rcu;
-	};
+	struct ns_tree;
+	struct rcu_head ns_rcu;
 };
 
 #define to_ns_common(__ns)                                    \
-- 
2.47.3