From 476ef286a7a5db8bda51325dc0c45887b4d99d9a Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Sat, 26 Sep 2026 15:44:24 +0200 Subject: [PATCH] namespace: reset the old parent's ->overmount in mnt_change_mountpoint() __umount_mnt() resets ->overmount of the parent when it unmounts the mount that overmounts it. mnt_change_mountpoint() doesn't. It moves the mount to its new parent and leaves the old one pointing at it. For a mount that is moved beneath in attach_recursive_mnt() that doesn't matter. The new mount has already reset ->overmount of the old parent by the time the old top mount is moved onto it. It matters for reparent() though. propagate_umount() moves a surviving overmount onto the first ancestor that isn't going to be unmounted. The mount it was taken from is unmounted immediately afterwards. If a file descriptor keeps that unmounted mount alive ->overmount points at the moved mount. That's a dangling pointer and UAF as the moved mount is unmounted and freed. do_lock_mount() follows ->overmount from the target path under mount_lock. With a file descriptor on such a mount as the target of a mount beneath topmost_overmount() walks into the freed mount and do_lock_mount() takes references on whatever it finds there. So reset ->overmount of the old parent in mnt_change_mountpoint() the same way __umount_mnt() does. A mount's ->overmount is then always a mount that is attached to it. Fixes: ffdc52fbbd58 ("prevent mount hash conflicts") Cc: stable@vger.kernel.org # v6.17+ Link: https://patch.msgid.link/20260926-work-mount-fixes-2-v1-1-f357abf3d17b@kernel.org Signed-off-by: Christian Brauner (Amutable) --- fs/namespace.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/fs/namespace.c b/fs/namespace.c index 73215f84e04d..230f95675a93 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1069,7 +1069,10 @@ static void attach_mnt(struct mount *mnt, struct mount *parent, void mnt_change_mountpoint(struct mount *parent, struct mountpoint *mp, struct mount *mnt) { struct mountpoint *old_mp = mnt->mnt_mp; + struct mount *old_parent = mnt->mnt_parent; + if (old_parent->overmount == mnt) + old_parent->overmount = NULL; list_del_init(&mnt->mnt_child); hlist_del_init(&mnt->mnt_mp_list); hlist_del_init_rcu(&mnt->mnt_hash);