From: Christian Brauner <christian@brauner.io>
Date: Mon, 17 Jun 2019 21:22:14 +0000 (+0200)
Subject: fs/namespace: fix unprivileged mount propagation
X-Git-Tag: v5.1.15~9
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4bb1bedcf3479b05567513ee21734524b838d794;p=thirdparty%2Fkernel%2Fstable.git

fs/namespace: fix unprivileged mount propagation

commit d728cf79164bb38e9628d15276e636539f857ef1 upstream.

When propagating mounts across mount namespaces owned by different user
namespaces it is not possible anymore to move or umount the mount in the
less privileged mount namespace.

Here is a reproducer:

  sudo mount -t tmpfs tmpfs /mnt
  sudo --make-rshared /mnt

  # create unprivileged user + mount namespace and preserve propagation
  unshare -U -m --map-root --propagation=unchanged

  # now change back to the original mount namespace in another terminal:
  sudo mkdir /mnt/aaa
  sudo mount -t tmpfs tmpfs /mnt/aaa

  # now in the unprivileged user + mount namespace
  mount --move /mnt/aaa /opt

Unfortunately, this is a pretty big deal for userspace since this is
e.g. used to inject mounts into running unprivileged containers.
So this regression really needs to go away rather quickly.

The problem is that a recent change falsely locked the root of the newly
added mounts by setting MNT_LOCKED. Fix this by only locking the mounts
on copy_mnt_ns() and not when adding a new mount.

Fixes: 3bd045cc9c4b ("separate copying and locking mount tree on cross-userns copies")
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@vger.kernel.org>
Tested-by: Christian Brauner <christian@brauner.io>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Christian Brauner <christian@brauner.io>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

diff --git a/fs/namespace.c b/fs/namespace.c
index c9cab307fa77d..061f247a3cdb5 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2079,6 +2079,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		/* Notice when we are propagating across user namespaces */
 		if (child->mnt_parent->mnt_ns->user_ns != user_ns)
 			lock_mnt_tree(child);
+		child->mnt.mnt_flags &= ~MNT_LOCKED;
 		commit_tree(child);
 	}
 	put_mountpoint(smp);
diff --git a/fs/pnode.c b/fs/pnode.c
index 7ea6cfb650778..012be405fec0f 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -262,7 +262,6 @@ static int propagate_one(struct mount *m)
 	child = copy_tree(last_source, last_source->mnt.mnt_root, type);
 	if (IS_ERR(child))
 		return PTR_ERR(child);
-	child->mnt.mnt_flags &= ~MNT_LOCKED;
 	mnt_set_mountpoint(m, mp, child);
 	last_dest = m;
 	last_source = child;