From a2b42babc912e4fb428cbb42adb4f6e1e067113d Mon Sep 17 00:00:00 2001 From: Christian Brauner Date: Wed, 23 Sep 2026 14:27:55 +0200 Subject: [PATCH] mount: refuse MOVE_MOUNT_SET_GROUP on an unbindable mount do_set_group() only accepts mounts as targets that are neither shared nor a slave. That encompasses undindable mounts. If the source mount is a slave mount the target mount will end up with source's master as its master. It keeps T_UNBINDABLE. That means we get a mount that is both unbindable and a slave: mount --bind /tmp/src /tmp/src mount --make-shared /tmp/src mount --bind /tmp/src /tmp/b mount --make-slave /tmp/b mount --bind /tmp/src /tmp/c mount --make-unbindable /tmp/c move_mount(b, "", c, "", MOVE_MOUNT_SET_GROUP) grep /tmp/c /proc/self/mountinfo ... /tmp/c ... master:646 unbindable ... This property cannot be produced any other way. change_mnt_propagation() drops the master when it makes a mount unbindable (iow, it becomes private) and doesn't touch an unbindable mount when it is supposed to turned into a slave mount. End-result is that the unbindable-slave mount receives everything propagated from its master's peer group while it can't be bind mounted itself. Not sure what that's supposed to do. On the other side: if the source mount is shared, the target mount drops T_UNBINDABLE because because set_mnt_shared() clears T_SHARED_MASK. So teach do_set_group() to refuse an unbindable target mount the same way a shared or slave target mount is refused. The main user of MOVE_MOUNT_SET_GROUP is CRIU which restores sharing first and applies MS_UNBINDABLE afterwards. It never hits this. Link: https://patch.msgid.link/20260923-work-mount-fixes-v1-3-f424cf8d3242@kernel.org Fixes: 9ffb14ef61ba ("move_mount: allow to add a mount into an existing group") Signed-off-by: Christian Brauner (Amutable) --- fs/namespace.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/namespace.c b/fs/namespace.c index a052f847c5df..d842fc1f1f1a 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -3468,7 +3468,7 @@ static int do_set_group(const struct path *from_path, const struct path *to_path return -EINVAL; /* Setting sharing groups is only allowed on private mounts */ - if (IS_MNT_SHARED(to) || IS_MNT_SLAVE(to)) + if (IS_MNT_SHARED(to) || IS_MNT_SLAVE(to) || IS_MNT_UNBINDABLE(to)) return -EINVAL; /* From should not be private */