linux-kernel - Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special files if caller has CAP_SYS

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Message-ID: <YNtLtkkDMWye485A@work-vm>
Date:   Tue, 29 Jun 2021 17:35:02 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>, dwalsh@...hat.com,
        "Schaufler, Casey" <casey.schaufler@...el.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "virtio-fs@...hat.com" <virtio-fs@...hat.com>,
        "berrange@...hat.com" <berrange@...hat.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        "selinux@...r.kernel.org" <selinux@...r.kernel.org>
Subject: Re: [RFC PATCH 0/1] xattr: Allow user.* xattr on symlink/special
 files if caller has CAP_SYS_RESOURCE

* Casey Schaufler (casey@...aufler-ca.com) wrote:
> On 6/29/2021 8:20 AM, Vivek Goyal wrote:
> > On Tue, Jun 29, 2021 at 07:38:15AM -0700, Casey Schaufler wrote:
> >
> > [..]
> >>>>>> User xattrs are less protected than security xattrs. You are exposing the
> >>>>>> security xattrs on the guest to the possible whims of a malicious, unprivileged
> >>>>>> actor on the host. All it needs is the right UID.
> >>>>> Yep, we realise that; but when you're mainly interested in making sure
> >>>>> the guest can't attack the host, that's less worrying.
> >>>> That's uncomfortable.
> >>> Why exactly?
> >> If a mechanism is designed with a known vulnerability you
> >> fail your validation/evaluation efforts.
> > We are working with the constraint that shared directory should not be
> > accessible to unpriviliged users on host. And with that constraint, what
> > you are referring to is not a vulnerability.
> 
> Sure, that's quite reasonable for your use case. It doesn't mean
> that the vulnerability doesn't exist, it means you've mitigated it. 
> 
> 
> >> Your mechanism is
> >> less general because other potential use cases may not be
> >> as cavalier about the vulnerability.
> > Prefixing xattrs with "user.virtiofsd" is just one of the options.
> > virtiofsd has the capability to prefix "trusted.virtiofsd" as well.
> > We have not chosen that because we don't want to give it CAP_SYS_ADMIN.
> >
> > So other use cases which don't like prefixing "user.virtiofsd", can
> > give CAP_SYS_ADMIN and work with it.
> >
> >> I think that you can
> >> approach this differently, get a solution that does everything
> >> you want, and avoid the known problem.
> > What's the solution? Are you referring to using "trusted.*" instead? But
> > that has its own problem of giving CAP_SYS_ADMIN to virtiofsd.
> 
> I'm coming to the conclusion that xattr namespaces, analogous
> to user namespaces, are the correct solution. They generalize
> for multiple filesystem and LSM use cases. The use of namespaces
> is well understood, especially in the container community. It
> looks to me as if it would address your use case swimmingly.

Yeh; although the details of getting the semantics right is tricky;
in particular, the stuff which clears capabilitiies/setuid/etc on writes
- should it clear xattrs that represent capabilities?  If the host
  performs a write, should it clear mapped xattrs capabilities?  If the
namespace performs a write should it clear just the mapped ones or the
host ones as well?  Our virtiofsd code performs acrobatics to make
sure they get cleared on write that are painful.

Dave

> >
> > Thanks
> > Vivek
> >
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK