Re: [PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

From: Andy Lutomirski
Date: Tue Oct 14 2014 - 18:20:11 EST

On Tue, Oct 14, 2014 at 3:14 PM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote:
> Quoting Serge E. Hallyn (serge@xxxxxxxxxx):
>> Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>> > Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
>> >
>> > > If a process gets access to a mount from a descendent or unrelated
>> > > user namespace, that process should not be able to take advantage of
>> > > setuid files or selinux entrypoints from that filesystem.
>> > >
>> > > This will make it safer to allow more complex filesystems to be
>> > > mounted in non-root user namespaces.
>> > >
>> > > This does not remove the need for MNT_LOCK_NOSUID. The setuid,
>> > > setgid, and file capability bits can no longer be abused if code in
>> > > a user namespace were to clear nosuid on an untrusted filesystem,
>> > > but this patch, by itself, is insufficient to protect the system
>> > > from abuse of files that, when execed, would increase MAC privilege.
>> > >
>> > > As a more concrete explanation, any task that can manipulate a
>> > > vfsmount associated with a given user namespace already has
>> > > capabilities in that namespace and all of its descendents. If they
>> > > can cause a malicious setuid, setgid, or file-caps executable to
>> > > appear in that mount, then that executable will only allow them to
>> > > elevate privileges in exactly the set of namespaces in which they
>> > > are already privileges.
>> > >
>> > > On the other hand, if they can cause a malicious executable to
>> > > appear with a dangerous MAC label, running it could change the
>> > > caller's security context in a way that should not have been
>> > > possible, even inside the namespace in which the task is confined.
>> >
>> > As presented this is complete and total nonsense. Mount propgation
>> > strongly weakens if not completely breaks the assumptions you are making
>> > in this code.
>> >
>> > To write any generic code that knows anything we need to capture a user
>> > namespace on struct super.
>> >
>> > Further I think all we really want is to filter out security labels from
>> > unprivileged mounts. uids/gids and the like should be completely fine
>> > because of the uid mappings.
>> >
>> > Having been down the route of comparing uids as userns uid tuples I am
>> > convinced that anything requires us to take the user namespace into
>> > account on a routine basis in the core will simply be broken for someone
>> > forgetting somewhere. This looks like a design that has that kind of
>> > susceptibility.
>> The above paragraph is very compelling. However Andy's patch is a step
>> in the right direction from what we've got. I think given what you say
>> below and given Andy's rationale above, simply tweaking his patch to
>> ignore the parent-userns loop, and return false if current_user_ns() !=
>> mount_userns, should be right? It'll prevent a child userns from
>> setting a selinux/apparmor entrypoint or POSIX file capabilities on a
>> file and having the parent userns trip over those.
> Ok, Andy's fn does the opposite, which will protect the parent userns,
> which is good.
> I suspect simply insisting that the user_ns's be equal is still better.
> It fits better with the idea that POSIX caps (and LSM entrypoints) are
> orthogonal to DAC. Kinda.

We could tighten it even further if we compared *mount* namespaces
instead of user namespaces. That would benefit Docker, non-userns-lxc
and such, too (sigh).

Actually, I see to good reason to insist on userns equality but not on
mountns equality. If we're not going to trust executables in foreign
namespaces, let's go all the way to distrust executables in all
foreign namespaces, at least unless someone thinks of a reason this
would break existing userspace.

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