libfs: fix get_stashed_dentry()

get_stashed_dentry() tries to optimistically retrieve a stashed dentry
from a provided location.  It needs to ensure to hold rcu lock before it
dereference the stashed location to prevent UAF issues.  Use
rcu_dereference() instead of READ_ONCE() it's effectively equivalent
with some lockdep bells and whistles and it communicates clearly that
this expects rcu protection.

Link: https://lore.kernel.org/r/20240906-vfs-hotfix-5959800ffa68@brauner
Fixes: 07fd7c32 ("libfs: add path_from_stashed()")
Reported-by: syzbot+f82b36bffae7ef78b6a7@syzkaller.appspotmail.com
Fixes: syzbot+f82b36bffae7ef78b6a7@syzkaller.appspotmail.com
Reported-by: syzbot+cbe4b96e1194b0e34db6@syzkaller.appspotmail.com
Fixes: syzbot+cbe4b96e1194b0e34db6@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/libfs.c

@@ -2117,12 +2117,12 @@ struct timespec64 simple_inode_init_ts(struct inode *inode)
 }
 EXPORT_SYMBOL(simple_inode_init_ts);
 
-static inline struct dentry *get_stashed_dentry(struct dentry *stashed)
+static inline struct dentry *get_stashed_dentry(struct dentry **stashed)
 {
 	struct dentry *dentry;
 
 	guard(rcu)();
-	dentry = READ_ONCE(stashed);
+	dentry = rcu_dereference(*stashed);
 	if (!dentry)
 		return NULL;
 	if (!lockref_get_not_dead(&dentry->d_lockref))
@@ -2219,7 +2219,7 @@ int path_from_stashed(struct dentry **stashed, struct vfsmount *mnt, void *data,
 	const struct stashed_operations *sops = mnt->mnt_sb->s_fs_info;
 
 	/* See if dentry can be reused. */
-	path->dentry = get_stashed_dentry(*stashed);
+	path->dentry = get_stashed_dentry(stashed);
 	if (path->dentry) {
 		sops->put_data(data);
 		goto out_path;