linux-kernel - [PATCH 3.13.y-ckt 71/95] RCU pathwalk breakage when running into a symlink overmounting something

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Message-Id: <1432753243-16151-72-git-send-email-kamal@canonical.com>
Date:	Wed, 27 May 2015 12:00:19 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 3.13.y-ckt 71/95] RCU pathwalk breakage when running into a symlink overmounting something

3.13.11-ckt21 -stable review patch.  If anyone has any objections, please let me know.

------------------

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

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: Kamal Mostafa <kamal@...onical.com>
---
 fs/namei.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index ddb6721..f804083 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1537,7 +1537,8 @@ 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;
 			}
@@ -2992,7 +2993,8 @@ 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;
 			}
-- 
1.9.1

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