mm: shrinker: fix shrinker_info teardown race with expansion

commit 65476d3 upstream.

expand_shrinker_info() iterates all visible memcgs under shrinker_mutex,
including memcgs that have not finished ->css_online() yet.

Once pn->shrinker_info has been published, teardown must stay serialized
with expand_shrinker_info() until that memcg is either fully online or no
longer visible to iteration.  Today alloc_shrinker_info() breaks that rule
by dropping shrinker_mutex before freeing a partially initialized
shrinker_info array, which may cause the following race:

CPU0                   CPU1
====                   ====

css_create
--> list_add_tail_rcu(&css->sibling, &parent_css->children);
    online_css
    --> mem_cgroup_css_online
        --> alloc_shrinker_info
            --> alloc node0 info
                rcu_assign_pointer(C->node0->shrinker_info, old0)
                alloc node1 info -> FAIL -> goto err
                mutex_unlock(shrinker_mutex)

                       shrinker_alloc()
                       --> shrinker_memcg_alloc
                           --> mutex_lock(shrinker_mutex)
                               expand_shrinker_info
                               --> mem_cgroup_iter see the memcg
                                   expand_one_shrinker_info
                                   --> old0 = C->node0->shrinker_info
                                       memcpy(new->unit, old0->unit, ...);

                free_shrinker_info
                --> kvfree(old0);

                                       /* double free !! */
                                       kvfree_rcu(old0, rcu);

The same problem exists later in mem_cgroup_css_online().  If
alloc_shrinker_info() succeeds but a subsequent objcg allocation fails,
the free_objcg -> free_shrinker_info() unwind path tears down the already
published pn->shrinker_info arrays without shrinker_mutex.  The
expand_one_shrinker_info() can race with that teardown in the same way,
leading to use-after-free or double-free of the old shrinker_info.

Fix this by serializing shrinker_info teardown with shrinker_mutex, and by
keeping alloc_shrinker_info() error cleanup inside the locked section.

Link: https://lore.kernel.org/20260617085658.27096-1-qi.zheng@linux.dev
Fixes: 307bece ("mm: shrinker: add a secondary array for shrinker_info::{map, nr_deferred}")
Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: Muchun Song <muchun.song@linux.dev>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Qi Zheng

mm/shrinker.c

@@ -59,12 +59,14 @@ static inline int shrinker_unit_alloc(struct shrinker_info *new,
 	return 0;
 }
 
-void free_shrinker_info(struct mem_cgroup *memcg)
+static void __free_shrinker_info(struct mem_cgroup *memcg)
 {
 	struct mem_cgroup_per_node *pn;
 	struct shrinker_info *info;
 	int nid;
 
+	lockdep_assert_held(&shrinker_mutex);
+
 	for_each_node(nid) {
 		pn = memcg->nodeinfo[nid];
 		info = rcu_dereference_protected(pn->shrinker_info, true);
@@ -74,6 +76,13 @@ void free_shrinker_info(struct mem_cgroup *memcg)
 	}
 }
 
+void free_shrinker_info(struct mem_cgroup *memcg)
+{
+	mutex_lock(&shrinker_mutex);
+	__free_shrinker_info(memcg);
+	mutex_unlock(&shrinker_mutex);
+}
+
 int alloc_shrinker_info(struct mem_cgroup *memcg)
 {
 	int nid, ret = 0;
@@ -98,8 +107,8 @@ int alloc_shrinker_info(struct mem_cgroup *memcg)
 	return ret;
 
 err:
+	__free_shrinker_info(memcg);
 	mutex_unlock(&shrinker_mutex);
-	free_shrinker_info(memcg);
 	return -ENOMEM;
 }