Re: [PATCH 1/2] dccp: ccid: move timers to struct dccp_sock

Jakub Kicinski
2020-11-09T17:49:43Z

On Mon, 9 Nov 2020 08:48:28 -0300 Thadeu Lima de Souza Cascardo wrote:
> On Fri, Oct 16, 2020 at 03:30:16PM -0700, Jakub Kicinski wrote:
> > On Tue, 13 Oct 2020 19:18:48 +0200 Kleber Sacilotto de Souza wrote:
> > > From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> > >
> > > When dccps_hc_tx_ccid is freed, ccid timers may still trigger. The reason
> > > del_timer_sync can't be used is because this relies on keeping a reference
> > > to struct sock. But as we keep a pointer to dccps_hc_tx_ccid and free that
> > > during disconnect, the timer should really belong to struct dccp_sock.
> > >
> > > This addresses CVE-2020-16119.
> > >
> > > Fixes: 839a6094140a (net: dccp: Convert timers to use timer_setup())
> > > Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
> > > Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
> >
> > I've been mulling over this fix.
> >
> > The layering violation really doesn't sit well.
> >
> > We're reusing the timer object. What if we are really unlucky, the
> > fires and gets blocked by a cosmic ray just as it's about to try to
> > lock the socket, then user manages to reconnect, and timer starts
> > again. Potentially with a different CCID algo altogether?
> >
> > Is disconnect ever called under the BH lock? Maybe plumb a bool
> > argument through to ccid*_hc_tx_exit() and do a sk_stop_timer_sync()
> > when called from disconnect()?
> >
> > Or do refcounting on ccid_priv so that the timer holds both the socket
> > and the priv?
>
> Sorry about too late a response. I was on vacation, then came back and spent a
> couple of days testing this further, and had to switch to other tasks.
>
> So, while testing this, I had to resort to tricks like having a very small
> expire and enqueuing on a different CPU. Then, after some minutes, I hit a UAF.
> That's with or without the first of the second patch.
>
> I also tried to refcount ccid instead of the socket, keeping the timer on the
> ccid, but that still hit the UAF, and that's when I had to switch tasks.

Hm, not instead, as well. I think trying cancel the timer _sync from
the disconnect path would be the simplest solution, tho.

> Oh, and in the meantime, I found one or two other fixes that we
> should apply, will send them shortly.
>
> But I would argue that we should apply the revert as it addresses the
> CVE, without really regressing the other UAF, as I argued. Does that
> make sense?

We can - it's always a little strange to go from one bug to a different
without a fix - but the justification being that while the previous UAF
required a race condition the new one is actually worst because it can
be triggered reliably?