mm/damon/dbgfs: Protect targets destructions with kdamond_lock

DAMON debugfs interface iterates current monitoring targets in 'dbgfs_target_ids_read()' while holding the corresponding 'kdamond_lock'. However, it also destructs the monitoring targets in 'dbgfs_before_terminate()' without holding the lock. This can result in a use_after_free bug. This commit avoids the race by protecting the destruction with the corresponding 'kdamond_lock'.

---
This cannot cleanly applied on 5.15.y tree. I will post a backport as soon as this is applied on the mainline.

 mm/damon/dbgfs.c | 2 ++
 1 file changed, 2 insertions(+)

index 58dbb9692279..489be9c830c4 100644
@@ -659,10 +659,12 @@ static void dbgfs_before_terminate(struct damon_ctx *ctx)
 	if (!targetid_is_pid(ctx))
 		return;
 
+	mutex_lock(&ctx->kdamond_lock);
 	damon_for_each_target_safe(t, next, ctx) {
 		put_pid((struct pid *)t->id);
 		damon_destroy_target(t);
 	}
+	mutex_unlock(&ctx->kdamond_lock);
 }
 
 static struct damon_ctx *dbgfs_new_ctx(void)

2.17.1