Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns

[Andy Lutomirski]

On Tue, Nov 18, 2014 at 9:13 AM, Seth Forshee
<seth.forshee@xxxxxxxxxxxxx> wrote:
> On Tue, Nov 18, 2014 at 09:09:34AM -0800, Andy Lutomirski wrote:
>> On Tue, Nov 18, 2014 at 7:21 AM, Seth Forshee
>> <seth.forshee@xxxxxxxxxxxxx> wrote:
>> > On Wed, Nov 12, 2014 at 10:22:54AM -0600, Seth Forshee wrote:
>> >> On Wed, Nov 12, 2014 at 02:09:15PM +0100, Miklos Szeredi wrote:
>> >> > On Tue, Nov 11, 2014 at 09:37:10AM -0600, Eric W. Biederman wrote:
>> >> >
>> >> > > > Maybe I'm being dense, but can someone give a concrete example of such an
>> >> > > > attack?
>> >> > >
>> >> > > There are two variants of things at play here.
>> >> > >
>> >> > > There is the classic if you don't freeze your context at open time when
>> >> > > you pass that file descriptor to another process unexpected things can
>> >> > > happen.
>> >> > >
>> >> > > An essentially harmless but extremely confusing example is what happens
>> >> > > to a partial read when it stops halfway through a uid value and the next
>> >> > > read on the same file descriptor is from a process in a different user
>> >> > > namespace.  Which uid value should be returned to userspace.
>> >> >
>> >> > Fuse device doesn't currently do partial reads, so that's a non-issue.
>> >> >
>> >> > > Now if I am in a nefarious mood I can create a unprivileged user
>> >> > > namespace, open /dev/fuse and mount a fuse filesystem.  Pass the file
>> >> > > descriptor to /dev/fuse to a processes that is in the default user
>> >> > > namespace (and thus can use any uid/gid).   With that file desctipor
>> >> > > report that there is a setuid 0 exectuable on that file system.
>> >> >
>> >> > Yes, and this would also be prevented by MNT_NOSUID, which would be a good idea
>> >> > anyway.  I just don't see the reason we'd want to allow clearing MNT_NOSUID in a
>> >> > private namespace.
>> >> >
>> >> > So we don't currently see a use case for relaxing either the MNT_NOSUID
>> >> > restriction or for relaxing the requirement on the user namespace the fuse
>> >> > server is in.  Is that correct?
>> >> >
>> >> > If so, we should leave both restrictions in place since that allows the greatest
>> >> > flexibility in the future, is either of those needs to be relaxed.
>> >>
>> >> I'm not aware of specific use cases for either at this point. However,
>> >> Andy's patch [1] will limit suid to the set of namespaces where the user
>> >> who mounted the filesystem already has privileges. Enforcing MNT_NOSUID
>> >> will require enforcement in the vfs, and in that case we definitely need
>> >> to decide whether the policy is to implicitly add the flag or fail the
>> >> mount attempt if the flag is not present [2].
>> >
>> > I asked around a bit, and it turns out there are use cases for nested
>> > containers (i.e. a container within a container) where the rootfs for
>> > the outer container mounts a filesystem containing the rootfs for the
>> > inner container. If that mount is nosuid then suid utilities like ping
>> > aren't going to work in the inner container.
>> >
>> > So since there's a use case for suid in a userns mount and we have what
>> > we belive are sufficient protections against using this as a vector to
>> > get privileges outside the container, I'm planning to move ahead without
>> > the MNT_NOSUID restriction. Any objections?
>>
>> Are you talking about MNT_NOSUID the flag or my ns-dependent thing?
>
> I'm talking about dropping the proposed requirement from Miklos that all
> fuse userns mounts are required to have the MNT_NOSUID flag. I intend to
> keep your ns-dependent thing.
>

In that case, I agree completely.  There are certainly uses for
non-nosuid mounts in containers, and I don't see why fuse should be
any different.

--Andy

> Thanks,
> Seth
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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