syzbot is reporting double kfree() at free_prealloced_shrinker() [1], for
destroy_unused_super() calls free_prealloced_shrinker() even if
prealloc_shrinker() returned an error. Explicitly clear shrinker name
when prealloc_shrinker() called kfree().

Link: https://syzkaller.appspot.com/bug?extid=8b481578352d4637f510 [1]
Reported-by: syzbot <syzbot+8b481578352d4637f510@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Fixes: e33c267ab70de424 ("mm: shrinkers: provide shrinkers with names")
---
 mm/vmscan.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 64ffdcd019a9..3302d9ed332e 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -707,8 +707,10 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 		return -ENOMEM;
 
 	err = __register_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 	return err;
 }
 #else
-- 
2.18.4

On Wed, Jul 20, 2022 at 11:47:55PM +0900, Tetsuo Handa wrote:
> syzbot is reporting double kfree() at free_prealloced_shrinker() [1], for
> destroy_unused_super() calls free_prealloced_shrinker() even if
> prealloc_shrinker() returned an error. Explicitly clear shrinker name
> when prealloc_shrinker() called kfree().
> 
> Link: https://syzkaller.appspot.com/bug?extid=8b481578352d4637f510 [1]
> Reported-by: syzbot <syzbot+8b481578352d4637f510@syzkaller.appspotmail.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Fixes: e33c267ab70de424 ("mm: shrinkers: provide shrinkers with names")

Hi Tetsuo!

Thank you for the fix! It looks good to me, however I think we should go a bit
further and and zero shrinker->name in all 4 cases where shrinker->name is
freed (an example code below).

Please, feel free to add my ack:
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!

---
 mm/shrinker_debug.c | 1 +
 mm/vmscan.c         | 9 +++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index e5b40c43221d..b05295bab322 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -251,6 +251,7 @@ void shrinker_debugfs_remove(struct shrinker *shrinker)
 	lockdep_assert_held(&shrinker_rwsem);
 
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 
 	if (!shrinker->debugfs_entry)
 		return;
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8fa03fb5c5c1..9e7d8db42918 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -647,8 +647,10 @@ int prealloc_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 		return -ENOMEM;
 
 	err = __prealloc_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 
 	return err;
 }
@@ -663,6 +665,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker)
 {
 #ifdef CONFIG_SHRINKER_DEBUG
 	kfree_const(shrinker->name);
+	shrinker->name = NULL;
 #endif
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
 		down_write(&shrinker_rwsem);
@@ -707,8 +710,10 @@ int register_shrinker(struct shrinker *shrinker, const char *fmt, ...)
 		return -ENOMEM;
 
 	err = __register_shrinker(shrinker);
-	if (err)
+	if (err) {
 		kfree_const(shrinker->name);
+		shrinker->name = NULL;
+	}
 	return err;
 }
 #else
-- 
2.36.1