Re: Steam is broken on new kernels

[Eric Dumazet]

On Wed, Jun 26, 2019 at 5:43 AM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> On 6/25/19 7:29 PM, Greg Kroah-Hartman wrote:
> > On Tue, Jun 25, 2019 at 07:02:20PM -0700, Guenter Roeck wrote:
> >> Hi Greg,
> >>
> >> On Sat, Jun 22, 2019 at 09:37:53AM +0200, Greg Kroah-Hartman wrote:
> >>> On Fri, Jun 21, 2019 at 10:28:21PM -0700, Linus Torvalds wrote:
> >>>> On Fri, Jun 21, 2019 at 6:03 PM Pierre-Loup A. Griffais
> >>>> <pgriffais@xxxxxxxxxxxxxxxxx> wrote:
> >>>>>
> >>>>> I applied Eric's path to the tip of the branch and ran that kernel and
> >>>>> the bug didn't occur through several logout / login cycles, so things
> >>>>> look good at first glance. I'll keep running that kernel and report back
> >>>>> if anything crops up in the future, but I believe we're good, beyond
> >>>>> getting distros to ship this additional fix.
> >>>>
> >>>> Good. It's now in my tree, so we can get it quickly into stable and
> >>>> then quickly to distributions.
> >>>>
> >>>> Greg, it's commit b6653b3629e5 ("tcp: refine memory limit test in
> >>>> tcp_fragment()"), and I'm building it right now and I'll push it out
> >>>> in a couple of minutes assuming nothing odd is going on.
> >>>
> >>> This looks good for 4.19 and 5.1, so I'll push out new stable kernels in
> >>> a bit for them.
> >>>
> >>> But for 4.14 and older, we don't have the "hint" to know this is an
> >>> outbound going packet and not to apply these checks at that point in
> >>> time, so this patch doesn't work.
> >>>
> >>> I'll see if I can figure anything else later this afternoon for those
> >>> kernels...
> >>>
> >>
> >> I may have missed it, but I don't see a fix for the problem in
> >> older stable branches. Any news ?
> >>
> >> One possibility might be be to apply the part of 75c119afe14f7 which
> >> introduces TCP_FRAG_IN_WRITE_QUEUE and TCP_FRAG_IN_RTX_QUEUE, if that
> >> is acceptable.
> >
> > That's what people have already discussed on the stable mailing list a
> > few hours ago, hopefully a patch shows up soon as I'm traveling at the
> > moment and can't do it myself...
> >
>
> Sounds good. Let me know if nothing shows up; I'll be happy to do it
> if needed.


Without the rb-tree for rtx queues, old kernels are vulnerable to SACK
attacks if sk_sndbuf is too big,
so I would simply  add a cushion in the test, instead of trying to
backport an illusion of the rb-tree fixes.



diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index a8772e11dc1cb42d4319b6fc072c625d284c7ad5..a554213afa4ac41120d781fe64b7cd18ff9b56e8
100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1274,7 +1274,7 @@ int tcp_fragment(struct sock *sk, struct sk_buff
*skb, u32 len,
        if (nsize < 0)
                nsize = 0;

-       if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf)) {
+       if (unlikely((sk->sk_wmem_queued >> 1) > sk->sk_sndbuf + 131072)) {
                NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
                return -ENOMEM;
        }