[PATCH 09/21] namespace: keep the lock on a mount that a propagated copy is moved beneath

[Christian Brauner <brauner@kernel.org>]

attach_recursive_mnt() transfers MNT_LOCKED from the top mount to the
mount that is moved beneath it with MOVE_MOUNT_BENEATH. This allows the
owner of a user namespace to replace its locked /proc or its root. The
mount beneath takes on the job of covering the underlying mountpoint
allowing the top mount to be unmounted.

Consider two mount namespaces:

(H) The host H has a shared mount P with a secret in P/d
(Z) Z is a user namespace made from M. Its copy of P receives
    propagation from the host's P and its copy of M's cover on P/d is
    locked Z's owner must not get to see P/d.

Now (H) mounts X on P/d. This propagates into (Z). The copy of X lands
beneath (Z)'s locked cover. The locked property is transfered from (Z)'s
cover to the copy of X propagated beneath it. The cover is now unlocked.

Now (H) unmounts X again. The copy of X in (Z) gets unmounted and the
covering mount is left unlocked on top of P/d. (Z) can now unmount it:

    Z: umount2(P/d)             = EINVAL            /* the cover is locked */
    H: mount X on P/d, umount X                     /* both propagate into Z /*
    Z: umount2(P/d)             = 0                 /* the cover is now unlocked */
    Z: read P/d/secret          = "covered-by-root" /* secret revealed */

So only transfer the locked property to the mount beneath for mounts the
caller has placed. A propagated copy that lands beneath a locked mount
is locked as well so that the mount at the bottom of the stack carries a
lock the way every check expects. The mount on top of it remains locked
to ensure that it keeps covering even if the propagated mount is
unmounted again.

Fixes: c62a4766937e ("move_mount: transfer MNT_LOCKED")
Cc: stable@vger.kernel.org # v7.1+
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/namespace.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index e74e63466c24..bb0183ec2aaf 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2712,15 +2712,17 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 			/*
 			 * If @q was locked it was meant to hide
 			 * whatever was under it. Let @child take over
-			 * that job and lock it, then we can unlock @q.
-			 * That'll allow another namespace to shed @q
-			 * and reveal @child. Clearly, that mounter
-			 * consented to this by not severing the mount
-			 * relationship. Otherwise, what's the point.
+			 * that job and lock it. If @child is the mount
+			 * the caller placed we can then unlock @q:
+			 * nothing another namespace does removes it
+			 * again. A propagated copy goes away when the
+			 * mounter of the original unmounts it, so @q
+			 * keeps its lock.
 			 */
 			if (IS_MNT_LOCKED(q)) {
 				child->mnt.mnt_flags |= MNT_LOCKED;
-				q->mnt.mnt_flags &= ~MNT_LOCKED;
+				if (child == source_mnt)
+					q->mnt.mnt_flags &= ~MNT_LOCKED;
 			}
 			mnt_change_mountpoint(r, mp, q);
 		}

-- 
2.53.0