Re: [PATCH 1/2] selinux: fix a race condition in security_read_policy()

[Ondrej Mosnacek <omosnace@redhat.com>]

On Mon, Aug 24, 2020 at 2:53 PM Stephen Smalley
<stephen.smalley.work@gmail.com> wrote:
> On Mon, Aug 24, 2020 at 8:47 AM Stephen Smalley
> <stephen.smalley.work@gmail.com> wrote:
> >
> > On Mon, Aug 24, 2020 at 7:30 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
> > >
> > > In security_read_policy(), the policy length is computed using
> > > security_policydb_len(), which does a separate transaction, and then
> > > another transaction is done to write the policydb into a buffer of this
> > > length.
> > >
> > > The bug is that the policy might be re-loaded in between the two
> > > transactions and so the length can be wrong. In case the new length is
> > > lower than the old length, the length is corrected at the end of the
> > > function. In case the new length is higher than the old one, an error is
> > > returned.
> > >
> > > Since we can't call vmalloc_user() under read_lock(), fix it by checking
> > > if the allocated buffer is sufficiently large after doing the allocation
> > > and taking the read lock and if not, retry the whole operation with the
> > > new size.
> > >
> > > Fixes: cee74f47a6ba ("SELinux: allow userspace to read policy back out of the kernel")
> > > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > > ---
> > >  security/selinux/ss/services.c | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > > index a48fc1b337ba9..2c9072f095985 100644
> > > --- a/security/selinux/ss/services.c
> > > +++ b/security/selinux/ss/services.c
> > > @@ -3849,14 +3849,22 @@ int security_read_policy(struct selinux_state *state,
> > >
> > >         *len = security_policydb_len(state);
> > >
> > > +again:
> > >         *data = vmalloc_user(*len);
> > >         if (!*data)
> > >                 return -ENOMEM;
> > >
> > > +       read_lock(&state->ss->policy_rwlock);
> > > +       if (*len < state->ss->policy->policydb.len) {
> > > +               *len = state->ss->policy->policydb.len;
> > > +               read_unlock(&state->ss->policy_rwlock);
> > > +               vfree(*data);
> > > +               goto again;
> > > +       }
> > > +
> > >         fp.data = *data;
> > >         fp.len = *len;
> > >
> > > -       read_lock(&state->ss->policy_rwlock);
> > >         rc = policydb_write(&state->ss->policy->policydb, &fp);
> > >         read_unlock(&state->ss->policy_rwlock);
> > >
> >
> > security_read_policy() is called with fsi->mutex held by selinuxfs, so
> > policy reload cannot occur in between the length computation and the
> > writing of the policydb.  Right?  It's another case where we could
> > pass down the mutex as in my rcu patches for a lockdep assertion.
>
> If my RCU patches are merged, we could modify security_read_policy()
> to take the mutex too and use rcu_dereference_protected() there,
> likewise getting rid of the separate security_policydb_len().  Or I
> could re-spin them to do that if any other changes are needed.
> Waiting to see if Paul wants any changes to either of those.

Oh, you're right, there is really no race condition thanks to the
mutex... So this patch isn't necessary then, but the second one should
still count as a simplification (with an updated commit message). I'm
fine with merging it into the RCU patch(es) if you want to do it. Or
I'll just send a rebased version if they get merged in the current
form.

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