From 3cab989afd8d8d1bc3d99fef0e7ed87c31e7b647 Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 24 Apr 2015 15:47:07 -0400
Subject: RCU pathwalk breakage when running into a symlink overmounting
 something

Calling unlazy_walk() in walk_component() and do_last() when we find
a symlink that needs to be followed doesn't acquire a reference to vfsmount.
That's fine when the symlink is on the same vfsmount as the parent directory
(which is almost always the case), but it's not always true - one _can_
manage to bind a symlink on top of something.  And in such cases we end up
with excessive mntput().

Cc: stable@vger.kernel.org # since 2.6.39
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index ffab2e0..4a8d998b 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1590,7 +1590,8 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
 
 	if (should_follow_link(path->dentry, follow)) {
 		if (nd->flags & LOOKUP_RCU) {
-			if (unlikely(unlazy_walk(nd, path->dentry))) {
+			if (unlikely(nd->path.mnt != path->mnt ||
+				     unlazy_walk(nd, path->dentry))) {
 				err = -ECHILD;
 				goto out_err;
 			}
@@ -3045,7 +3046,8 @@ finish_lookup:
 
 	if (should_follow_link(path->dentry, !symlink_ok)) {
 		if (nd->flags & LOOKUP_RCU) {
-			if (unlikely(unlazy_walk(nd, path->dentry))) {
+			if (unlikely(nd->path.mnt != path->mnt ||
+				     unlazy_walk(nd, path->dentry))) {
 				error = -ECHILD;
 				goto out;
 			}
-- 
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