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@xxxxxxxxxxxxx> > > > > > > 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@xxxxxxxxxxxxx> > > > Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@xxxxxxxxxxxxx> > > > > 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?
