netdev - Re: [PATCH net] net: enforce xmit_recursion for devices with a queue

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Date:   Thu, 14 Mar 2019 15:15:05 +0100
From:   Sabrina Dubroca <sd@...asysnail.net>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, Jianlin Shi <jishi@...hat.com>,
        Stefano Brivio <sbrivio@...hat.com>
Subject: Re: [PATCH net] net: enforce xmit_recursion for devices with a queue

2019-03-14, 05:58:03 -0700, Eric Dumazet wrote:
> 
> 
> On 03/14/2019 03:15 AM, Sabrina Dubroca wrote:
> > Commit 745e20f1b626 ("net: add a recursion limit in xmit path")
> > introduced a recursion limit, but it only applies to devices without a
> > queue. Virtual devices with a queue (either because they don't have
> > the IFF_NO_QUEUE flag, or because the administrator added one) can
> > still cause an unbounded recursion, via __dev_queue_xmit ->
> > __dev_xmit_skb -> qdisc_run -> __qdisc_run -> qdisc_restart ->
> > sch_direct_xmit -> dev_hard_start_xmit . Jianlin reported this in a
> > setup with 16 gretap devices stacked on top of one another.
> > 
> > This patch prevents the stack overflow by incrementing xmit_recursion in
> > code paths that can call dev_hard_start_xmit() (like commit 745e20f1b626
> > did). If the recursion limit is exceeded, the packet is enqueued and the
> > qdisc is scheduled.
> > 
> > Reported-by: Jianlin Shi <jishi@...hat.com>
> > Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> > Reviewed-by: Stefano Brivio <sbrivio@...hat.com>
> 
> Hi Sabrina, thanks for the patch.
> 
> Can't we detect this in the control path instead ?

I don't see how. You could have a perfectly reasonable set of gretap
devices that trigger this situation from simply reshuffling the IP
addresses:

gretap$x remote 1.1.$((x-1)).{1,2}
(all those addresses set on a single veth device)

Then you move those addresses to the corresponding device
(1.1.${x}.{1,2} on gretap$x), and your machine crashes.

-- 
Sabrina