netdev - Re: Spurious TCP retransmissions on ack vs kfree

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:   Fri, 26 Feb 2021 08:46:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: Spurious TCP retransmissions on ack vs kfree_skb reordering

On Fri, 26 Feb 2021 17:35:11 +0100 Eric Dumazet wrote:
> On Fri, Feb 26, 2021 at 5:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
> > On Fri, 26 Feb 2021 11:41:22 +0100 Eric Dumazet wrote:  
> > > Yes, this packetdrill test confirms TCP INFO stuff seems correct .  
> >
> > Looks like it's TcpExtTCPSpuriousRtxHostQueues - the TFO fails as it
> > might, but at the time the syn is still not kfree_skb()d because of
> > the IRQ coalescing settings, so __tcp_retransmit_skb() returns -EBUSY
> > and we have to wait for a timeout.
> >
> > Credit to Neil Spring @FB for figuring it out.  
> 
> Yes, this makes sense.
> 
> Presumably tcp_send_syn_data() could allocate a regular (non fclone)
> skb, to avoid this.
> 
> But if skb_still_in_host_queue() returns true, __tcp_retransmit_skb()
> should return -EBUSY
> and your tracepoint should not be called ?

Right, looking at the stack trace the call I was tracing comes later
from the RTO timer.

> In anycase, the bytes_acked should not be 742 as mentioned in your
> email, if only the SYN was acked ?

You're right, it's 1. I did a braino in the trace print yesterday.