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Date: Thu, 14 Mar 2019 07:56:10 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: Sabrina Dubroca <sd@...asysnail.net> 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 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 ? Thanks.
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