linux-kernel - [PATCH 3.12 072/142] RCU pathwalk breakage when running into a symlink overmounting something

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Message-Id: <6e310c82af1ba014a4bb312551d1a74d1c71761a.1431761807.git.jslaby@suse.cz>
Date:	Sat, 16 May 2015 09:37:12 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Jiri Slaby <jslaby@...e.cz>
Subject: [PATCH 3.12 072/142] RCU pathwalk breakage when running into a symlink overmounting something

From: Al Viro <viro@...iv.linux.org.uk>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 3cab989afd8d8d1bc3d99fef0e7ed87c31e7b647 upstream.

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().

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Jiri Slaby <jslaby@...e.cz>
---
 fs/namei.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1004966437f9..c0c78e193e2a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1557,7 +1557,8 @@ static inline int walk_component(struct nameidata *nd, struct path *path,
 
 	if (should_follow_link(inode, 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;
 			}
@@ -3023,7 +3024,8 @@ finish_lookup:
 
 	if (should_follow_link(inode, !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;
 			}
-- 
2.3.7

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