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>

diff --git a/fs/namei.c b/fs/namei.c
index ffab2e06e1472eca41449cbe5d8fe22baa219d29..4a8d998b7274b3406532cc012d05ee18aa9b5bba 100644 (file)
--- 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;
                        }