[PATCH] fs/namespace: fix unprivileged mount propagation

[PATCH] fs/namespace: fix unprivileged mount propagation

[Christian Brauner]

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>
---
 fs/namespace.c | 1 +
 fs/pnode.c     | 3 +--
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index b26778bdc236..44b540e6feb9 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2105,6 +2105,7 @@ 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 595857a1883e..d118106fa631 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -257,11 +257,10 @@ static int propagate_one(struct mount *m)
 		if (IS_MNT_SHARED(m))
 			type |= CL_MAKE_SHARED;
 	}
-		
+
 	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;
-- 
2.21.0

Re: [PATCH] fs/namespace: fix unprivileged mount propagation

[Linus Torvalds]

On Mon, Jun 17, 2019 at 11:47 AM Christian Brauner <christian@brauner.io> wrote:
>
> 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.

I will wait a short while in the hope of getting Al's ack for this,
but since it looks about as good as it likely can be, I suspect I'll
just apply it later today even without such an ack..

                    Linus

Re: [PATCH] fs/namespace: fix unprivileged mount propagation

[Christian Brauner]

On Mon, Jun 17, 2019 at 11:50:13AM -0700, Linus Torvalds wrote:
> On Mon, Jun 17, 2019 at 11:47 AM Christian Brauner <christian@brauner.io> wrote:
> >
> > 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.
> 
> I will wait a short while in the hope of getting Al's ack for this,
> but since it looks about as good as it likely can be, I suspect I'll
> just apply it later today even without such an ack..

Thanks!
Note that I stupidly messed up whitespace by accidently adding an
additional newline. I'll just send a v1 that fixes this nonsense.

Christian

Re: [PATCH] fs/namespace: fix unprivileged mount propagation

[Al Viro]

On Mon, Jun 17, 2019 at 11:50:13AM -0700, Linus Torvalds wrote:
> On Mon, Jun 17, 2019 at 11:47 AM Christian Brauner <christian@brauner.io> wrote:
> >
> > 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.
> 
> I will wait a short while in the hope of getting Al's ack for this,
> but since it looks about as good as it likely can be, I suspect I'll
> just apply it later today even without such an ack..

Give me a bit; I'm busy digging myself from under the pile of mail accumulated
in the last few weeks (bronchitis sucked, especially when it got to the point
where one can't stay asleep for more than an hour or get more than about 5 hours
per day total).  I'm trying not to throw anything relevant out, but if I don't
reply to something important today or tomorrow, please resend it my way -
the pile had been 36Kmail (down to 24K now) ;-/