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Date:	Tue, 14 Oct 2014 15:47:59 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel@...ts.sourceforge.net,
	Miklos Szeredi <miklos@...redi.hu>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Seth Forshee <seth.forshee@...onical.com>
Subject: Re: [PATCH] fs: Treat non-ancestor-namespace mounts as MNT_NOSUID

On Tue, Oct 14, 2014 at 3:45 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Andy Lutomirski (luto@...capital.net):
>> On Tue, Oct 14, 2014 at 3:14 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
>> > Quoting Serge E. Hallyn (serge@...lyn.com):
>> >> Quoting Eric W. Biederman (ebiederm@...ssion.com):
>> >> > Andy Lutomirski <luto@...capital.net> 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.
>
> I have no doubt there is code out there in production which ends up
> executing /proc/pid/root/sbin/ifconfig etc.  Cause, you know, you really
> wanna execute whatever garbage is there...  Breaking that might be a
> good thing.

Heh.

But it's the code that executes /proc/pid/root/sbin/sudo that we'll break :)

I'll send a new patch.

--Andy
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