Closing the BPF map permission loophole

[Paul Moore]

On Wed, Sep 28, 2022 at 7:24 AM Roberto Sassu
<roberto.sassu at huaweicloud.com> wrote
> On Wed, 2022-09-28 at 12:33 +0200, Toke Høiland-Jørgensen wrote:
> > Roberto Sassu <roberto.sassu at huaweicloud.com> writes:
> >
> > > On Wed, 2022-09-28 at 09:52 +0100, Lorenz Bauer wrote:
> > > > On Mon, 26 Sep 2022, at 17:18, Roberto Sassu wrote:
> > > > > Uhm, if I get what you mean, you would like to add DAC controls
> > > > > to
> > > > > the
> > > > > pinned map to decide if you can get a fd and with which modes.
> > > > >
> > > > > The problem I see is that a map exists regardless of the pinned
> > > > > path
> > > > > (just by ID).
> > > >
> > > > Can you spell this out for me? I imagine you're talking about
> > > > MAP_GET_FD_BY_ID, but that is CAP_SYS_ADMIN only, right? Not
> > > > great
> > > > maybe, but no gaping hole IMO.
> > >
> > > +linux-security-module ML (they could be interested in this topic
> > > as
> > > well)
> > >
> > > Good to know! I didn't realize it before.
> > >
> > > I figured out better what you mean by escalating privileges.
> > >
> > > Pin a read-only fd, get a read-write fd from the pinned path.
> > >
> > > What you want to do is, if I pin a read-only fd, I should get read-
> > > only
> > > fds too, right?
> > >
> > > I think here there could be different views. From my perspective,
> > > pinning is just creating a new link to an existing object.
> > > Accessing
> > > the link does not imply being able to access the object itself (the
> > > same happens for files).
> > >
> > > I understand what you want to achieve. If I have to choose a
> > > solution,
> > > that would be doing something similar to files, i.e. add owner and
> > > mode
> > > information to the bpf_map structure (m_uid, m_gid, m_mode). We
> > > could
> > > add the MAP_CHMOD and MAP_CHOWN operations to the bpf() system call
> > > to
> > > modify the new fields.
> > >
> > > When you pin the map, the inode will get the owner and mode from
> > > bpf_map. bpf_obj_get() will then do DAC-style verification similar
> > > to
> > > MAC-style verification (with security_bpf_map()).
> >
> > As someone pointed out during the discussing at LPC, this will
> > effectively allow a user to create files owned by someone else, which
> > is
> > probably not a good idea either from a security PoV. (I.e., user A
> > pins
> > map owned by user B, so A creates a file owned by B).
>
> Uhm, I see what you mean. Right, it is not a good idea, the owner of
> the file should the one that pinned the map.
>
> Other than that, DAC verification on the map would be still correct, as
> it would be independent from the DAC verification of the file.

I only became aware of this when the LSM list was CC'd so I'm a little
behind on what is going on here ... looking quickly through the
mailing list archive it looks like there is an issue with BPF map
permissions not matching well with their associated fd permissions,
yes?  From a LSM perspective, there are a couple of hooks that
currently use the fd's permissions (read/write) to determine the
appropriate access control check.

Is the plan to ensure that the map and fd permissions are correct at
the core BPF level, or do we need to do some additional checks in the
LSMs (currently only SELinux)?

-- 
paul-moore.com