Re: Steam is broken on new kernels

[Greg Kroah-Hartman]

On Wed, Jun 26, 2019 at 06:20:17AM +0200, Eric Dumazet wrote:
> 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;
>         }

That's a funny magic number, can we document what it means?

And yes, it's a much simpler patch, I'd rather take this than the fake
backport.

thanks,

greg k-h