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: [email protected] # since 2.6.39 Signed-off-by: Al Viro <[email protected]>
zarubaf · Apr 24, 2015 · 3cab989 · 3cab989
1 parent ac74d8d
commit 3cab989
Showing 1 changed file with 4 additions and 2 deletions.
diff --git 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 @@ static int do_last(struct nameidata *nd, struct path *path,
 
 	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;
 			}