fsnotify: fix UAF from FS_ERROR event on a shutting down filesystem

Protect against use after free when filesystem calls fsnotify_sb_error()
during fs shutdown.

Move freeing of sb->s_fsnotify_info to destroy_super_work(), because it
may be accessed from fs shutdown context.

Reported-by: syzbot+5e3f9b2a67b45f16d4e6@syzkaller.appspotmail.com
Suggested-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/linux-fsdevel/20240416173211.4lnmgctyo4jn5fha@quack3/
Fixes: 07a3b8d0 ("fsnotify: lazy attach fsnotify_sb_info state to sb")
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240416181452.567070-1-amir73il@gmail.com>

fs/notify/fsnotify.c

@@ -103,7 +103,11 @@ void fsnotify_sb_delete(struct super_block *sb)
 	WARN_ON(fsnotify_sb_has_priority_watchers(sb, FSNOTIFY_PRIO_CONTENT));
 	WARN_ON(fsnotify_sb_has_priority_watchers(sb,
 						  FSNOTIFY_PRIO_PRE_CONTENT));
-	kfree(sbinfo);
+}
+
+void fsnotify_sb_free(struct super_block *sb)
+{
+	kfree(sb->s_fsnotify_info);
 }
 
 /*

fs/super.c

@@ -274,6 +274,7 @@ static void destroy_super_work(struct work_struct *work)
 {
 	struct super_block *s = container_of(work, struct super_block,
 							destroy_work);
+	fsnotify_sb_free(s);
 	security_sb_free(s);
 	put_user_ns(s->s_user_ns);
 	kfree(s->s_subtype);

include/linux/fsnotify_backend.h

@@ -576,6 +576,7 @@ extern int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data
 extern void __fsnotify_inode_delete(struct inode *inode);
 extern void __fsnotify_vfsmount_delete(struct vfsmount *mnt);
 extern void fsnotify_sb_delete(struct super_block *sb);
+extern void fsnotify_sb_free(struct super_block *sb);
 extern u32 fsnotify_get_cookie(void);
 
 static inline __u32 fsnotify_parent_needed_mask(__u32 mask)
@@ -880,6 +881,9 @@ static inline void __fsnotify_vfsmount_delete(struct vfsmount *mnt)
 static inline void fsnotify_sb_delete(struct super_block *sb)
 {}
 
+static inline void fsnotify_sb_free(struct super_block *sb)
+{}
+
 static inline void fsnotify_update_flags(struct dentry *dentry)
 {}