[PATCH] vmscan: fix potential arbitrary pointer passed to kfree in unregister_shrinker

[tujinjiang@bytedance.com]

From: Jinjiang Tu <tujinjiang@bytedance.com>

when shrinker is registered with SHRINKER_MEMCG_AWARE flag,
register_shrinker will not initialize shrinker->nr_deferred,
but the pointer will be passed to kfree in unregister_shrinker
when the shrinker is unregistered. This leads to kernel crash
when the shrinker object is dynamically allocated.

To fix it, this patch initialize shrinker->nr_deferred at the
beginning of prealloc_shrinker.

Signed-off-by: Jinjiang Tu <tujinjiang@bytedance.com>
---
 mm/vmscan.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f7d9a683e3a7..06ab5a398971 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -613,6 +613,7 @@ int prealloc_shrinker(struct shrinker *shrinker)
 	unsigned int size;
 	int err;
 
+	shrinker->nr_deferred = NULL;
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		err = prealloc_memcg_shrinker(shrinker);
 		if (err != -ENOSYS)
-- 
2.17.1