[PATCH 3.10 59/65] RCU pathwalk breakage when running into a symlink overmounting something

Greg Kroah-Hartman
2015-05-02T19:52:06Z

From: Al Viro <viro@zeniv.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@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/namei.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1542,7 +1542,8 @@ static inline int walk_component(struct

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;
}
@@ -2824,7 +2825,8 @@ 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;
}