linux-kernel - [PATCH 3.4 055/146] RCU pathwalk breakage when running into a symlink overmounting something

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Date:	Tue, 15 Sep 2015 17:02:50 +0800
From:	lizf@...nel.org
To:	stable@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Zefan Li <lizefan@...wei.com>
Subject: [PATCH 3.4 055/146] RCU pathwalk breakage when running into a symlink overmounting something

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

3.4.109-rc1 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>
[lizf: Backported to 3.4: drop the changes to do_last()]
Signed-off-by: Zefan Li <lizefan@...wei.com>
---
 fs/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index bdcd705..9c4b9b8 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1314,7 +1314,8 @@ 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))) {
 				terminate_walk(nd);
 				return -ECHILD;
 			}
-- 
1.9.1

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