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

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Message-ID: <20190314174038.GA18679@bistromath.localdomain>
Date:   Thu, 14 Mar 2019 18:40:38 +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, 07:56:10 -0700, Eric Dumazet wrote:
> 
> 
> On 03/14/2019 07:15 AM, Sabrina Dubroca wrote:
> > 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.
> > 
> 
> If this only can be done with gretap, why gretap cant implement the protection,
> outside of the fast path ?

It's not just gretap. VXLAN will do the same as long as you add a
qdisc. I expect other types of tunnels to behave like that.

-- 
Sabrina