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

[Jann Horn]

On Tue, Jun 26, 2018 at 12:36 AM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>
> On Mon, Jun 25, 2018 at 12:34 PM Jann Horn <jannh@xxxxxxxxxx> 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.