[PATCH v3 0/1] Relax restrictions on user.* xattr

Dr. David Alan Gilbert dgilbert at redhat.com
Mon Sep 6 14:55:28 UTC 2021

* Casey Schaufler (casey at schaufler-ca.com) wrote:
> On 9/2/2021 1:06 PM, Vivek Goyal wrote:

> >  If LSMs are not configured,
> > then hiding the directory is the solution.
> It's not a solution at all. It's wishful thinking that
> some admin is going to do absolutely everything right, will
> never make a mistake and will never, ever, read the mount(2)
> man page.

That is why we run our virtiofsd with a sandbox setup and seccomp; and
frankly anything we can or could turn on we would.

> > So why that's not a solution and only relying on CAP_SYS_ADMIN is the
> > solution. I don't understand that part.
> It comes back to your design, which is fundamentally flawed. You
> can't store system security information in an attribute that can
> be manipulated by untrusted entities. That's why we have system.*
> xattrs. You want to have an attribute on the host that maps to a
> security attribute on the guest. The host has to protect the attribute
> on the guest with mechanisms of comparable strength as the guest's
> mechanisms.

Can you just explain this line to me a bit more: 
> Otherwise you can't trust the guest with host data.

Note we're not trying to trust the guest with the host data here;
we're trying to allow the guest to store the data on the host, while
trusting the host.

> It's a real shame that CAP_SYS_ADMIN is so scary. The capability
> mechanism as implemented today won't scale to the hundreds of individual
> capabilities it would need to break CAP_SYS_ADMIN up. Maybe someday.
> I'm not convinced that there isn't a way to accomplish what you're
> trying to do without privilege, but this isn't it, and I don't know
> what is. Sorry.
> > Also if directory is not hidden, unprivileged users can change file
> > data and other metadata.
> I assumed that you've taken that into account. Are you saying that
> isn't going to be done correctly either?
> >  Why that's not a concern and why there is
> > so much of focus only security xattr.
> As with an NFS mount, the assumption is that UID 567 (or its magically
> mapped equivalent) has the same access rights on both the server/host
> and client/guest. I'm not worried about the mode bits because they are
> presented consistently on both machines. If, on the other hand, an
> attribute used to determine access is security.esprit on the guest and
> user.security.esprit on the host, the unprivileged user on the host
> can defeat the privilege requirements on the guest. That's why.

We're OK with that; remember that the host can do wth it likes to the
guest anyway - it can just go in and poke at the guests RAM if it wants
to do something evil to the guest.
We wouldn't suggest using a scheme like this once you have
encrypted/protected guest RAM for example (SEV/TDX etc)

> >  If you were to block modification
> > of file then you will have rely on LSMs.
> No. We're talking about the semantics of the xattr namespaces.
> LSMs can further constrain access to xattrs, but the basic rules
> of access to the user.* and security.* attributes are different
> in any case. This is by design.

I'm happy if you can suggest somewhere else to store the guests xattr
data other than in one of the hosts xattr's - the challenge is doing
that in a non-racy way, and making sure that the xattr's never get
associated with the wrong file as seen by a guest.

> >  And if LSMs are not configured,
> > then we will rely on shared directory not being visible.
> LSMs are not the problem. LSMs use security.* xattrs, which is why
> they come up in the discussion.
> > Can you please help me understand why hiding shared directory from
> > unprivileged users is not a solution
> Maybe you can describe the mechanism you use to "hide" a shared directory
> on the host. If the filesystem is mounted on the host it seems unlikely
> that you can provide a convincing argument for sufficient protection.

Why? What can a guests fs mounted on the host, under one of the
directories that's already typically used for container fs's do - it's
already what fileservers, and existing container systems do.


> >  (With both LSMs configured or
> > not configured on host). That's a requirement for virtiofs anyway. 
> > And if we agree on that, then I don't see why using "user.*" xattrs
> > for storing guest sercurity attributes is a problem.
> >
> > Thanks
> > Vivek
> >
Dr. David Alan Gilbert / dgilbert at redhat.com / Manchester, UK

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