[Devel] [PATCH RHEL7 COMMIT] RCU pathwalk breakage when running into a symlink overmounting something

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.25.1.vz7.180.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.25.1.vz7.180.8
------>
commit e68cad95614d71ce3dae5027895e626383bbf225
Author: Al Viro <viro at zeniv.linux.org.uk>
Date:   Thu May 27 13:11:06 2021 +0300

    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 at vger.kernel.org # since 2.6.39
    Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
    
    --
    
    When symlink-symlink bindmount is created on ploop, which is posible
    after we've ported open_tree()+move_mount() kernel interface, calling
    lstat on it makes excess decrement of the bindmount's mnt_pcp refcount,
    which leads to leaked mount and thus blocks ploop device and thus
    blocks container. So after this symlink-symlink bindmount is added in
    container one can't restart this container.
    
    https://jira.sw.ru/browse/PSBM-129639
    (cherry-pick from ms 3cab989afd8d8d1bc3d99fef0e7ed87c31e7b647)
    Signed-off-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>
---
 fs/namei.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index a25c05a..85c0ffa 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1601,7 +1601,8 @@ 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;
 			}
@@ -3208,7 +3209,8 @@ 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;
 			}