linux-kernel - Re: [PATCH] selinux: move user accesses in selinuxfs out of locked regions

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Date:   Thu, 28 Jun 2018 20:15:27 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     jannh@...gle.com
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        Eric Paris <eparis@...isplace.org>, selinux@...ho.nsa.gov,
        security@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selinux: move user accesses in selinuxfs out of locked regions

On Mon, Jun 25, 2018 at 6:40 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Tue, Jun 26, 2018 at 12:36 AM Paul Moore <paul@...l-moore.com> wrote:
> >
> > On Mon, Jun 25, 2018 at 12:34 PM Jann Horn <jannh@...gle.com> wrote:
> > > If a user is accessing a file in selinuxfs with a pointer to a userspace
> > > buffer that is backed by e.g. a userfaultfd, the userspace access can
> > > stall indefinitely, which can block fsi->mutex if it is held.
> > >
> > > For sel_read_policy(), remove the locking, since this method doesn't seem
> > > to access anything that requires locking.
> >
> > Forgive me, I'm thinking about this quickly so I could be very wrong
> > here, but isn't the mutex needed to prevent problems in multi-threaded
> > apps hitting the same fd at the same time?
>
> sel_read_policy() operates on a read-only copy of the policy, accessed
> via ->private_data, allocated using vmalloc in sel_open_policy() via
> security_read_policy(). As far as I can tell, nothing can write to
> that read-only copy of the policy. None of the handlers in
> sel_policy_ops write - they just mmap as readonly (in which case
> you're already reading without locks, by the way) or read.

Great, thanks.

-- 
paul moore
www.paul-moore.com