From af17e1fc7cb773c7f22f7343323cf102ea52a50b Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 8 Nov 2019 22:08:29 -0500
Subject: [PATCH] exportfs_decode_fh(): negative pinned may become positive
 without the parent locked

[ Upstream commit a2ece088882666e1dc7113744ac912eb161e3f87 ]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/exportfs/expfs.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 63707abcbeb3..808cae6d5f50 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -517,26 +517,33 @@ struct dentry *exportfs_decode_fh(struct vfsmount *mnt, struct fid *fid,
 		 * inode is actually connected to the parent.
 		 */
 		err = exportfs_get_name(mnt, target_dir, nbuf, result);
-		if (!err) {
-			inode_lock(target_dir->d_inode);
-			nresult = lookup_one_len(nbuf, target_dir,
-						 strlen(nbuf));
-			inode_unlock(target_dir->d_inode);
-			if (!IS_ERR(nresult)) {
-				if (nresult->d_inode) {
-					dput(result);
-					result = nresult;
-				} else
-					dput(nresult);
-			}
+		if (err) {
+			dput(target_dir);
+			goto err_result;
 		}
 
+		inode_lock(target_dir->d_inode);
+		nresult = lookup_one_len(nbuf, target_dir, strlen(nbuf));
+		if (!IS_ERR(nresult)) {
+			if (unlikely(nresult->d_inode != result->d_inode)) {
+				dput(nresult);
+				nresult = ERR_PTR(-ESTALE);
+			}
+		}
+		inode_unlock(target_dir->d_inode);
 		/*
 		 * At this point we are done with the parent, but it's pinned
 		 * by the child dentry anyway.
 		 */
 		dput(target_dir);
 
+		if (IS_ERR(nresult)) {
+			err = PTR_ERR(nresult);
+			goto err_result;
+		}
+		dput(result);
+		result = nresult;
+
 		/*
 		 * And finally make sure the dentry is actually acceptable
 		 * to NFSD.