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From:   Ondrej Mosnacek <omosnace@redhat.com>
Date:   Mon, 24 Aug 2020 22:01:17 +0200
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Subject: Re: [PATCH] SELinux: Measure state and hash of policy using IMA
To:     Stephen Smalley <stephen.smalley.work@gmail.com>
Cc:     Lakshmi Ramasubramanian <nramas@linux.microsoft.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Casey Schaufler <casey@schaufler-ca.com>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        tusharsu@linux.microsoft.com, Sasha Levin <sashal@kernel.org>,
        James Morris <jmorris@namei.org>,
        linux-integrity@vger.kernel.org,
        SElinux list <selinux@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Mon, Aug 24, 2020 at 9:30 PM Stephen Smalley
<stephen.smalley.work@gmail.com> wrote:
> On Mon, Aug 24, 2020 at 2:13 PM Lakshmi Ramasubramanian
> <nramas@linux.microsoft.com> wrote:
> >
> > On 8/24/20 7:00 AM, Stephen Smalley wrote:
> >
> > >> +int security_read_policy_kernel(struct selinux_state *state,
> > >> +                               void **data, size_t *len)
> > >> +{
> > >> +       int rc;
> > >> +
> > >> +       rc = security_read_policy_len(state, len);
> > >> +       if (rc)
> > >> +               return rc;
> > >> +
> > >> +       *data = vmalloc(*len);
> > >> +       if (!*data)
> > >> +               return -ENOMEM;
> > >>
> > >> +       return security_read_selinux_policy(state, data, len);
> > >>   }
> > >
> > > See the discussion here:
> > > https://lore.kernel.org/selinux/20200824113015.1375857-1-omosnace@redhat.com/T/#t
> > >
> > > In order for this to be safe, you need to ensure that all callers of
> > > security_read_policy_kernel() have taken fsi->mutex in selinuxfs and
> > > any use of security_read_policy_len() occurs while holding the mutex.
> > > Otherwise, the length can change between security_read_policy_len()
> > > and security_read_selinux_policy() if policy is reloaded.
> > >
> >
> > "struct selinux_fs_info" is available when calling
> > security_read_policy_kernel() - currently in measure.c.
> > Only "struct selinux_state" is.
> >
> > Is Ondrej's re-try approach I need to use to workaround policy reload issue?
>
> No, I think perhaps we should move the mutex to selinux_state instead
> of selinux_fs_info.  selinux_fs_info has a pointer to selinux_state so
> it can then use it indirectly.  Note that your patches are going to
> conflict with other ongoing work in the selinux next branch that is
> refactoring policy load and converting the policy rwlock to RCU.

Yeah, and I'm experimenting with a patch on top of Stephen's RCU work
that would allow you to do this in a straightforward way without even
messing with the fsi->mutex. My patch may or may not be eventually
committed, but either way I'd recommend holding off on this for a
while until the dust settles around the RCU conversion.

-- 
Ondrej Mosnacek
Software Engineer, Platform Security - SELinux kernel
Red Hat, Inc.