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>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/namei.c b/fs/namei.c
index ffab2e06e147..4a8d998b7274 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;
                         }