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Date: Thu, 14 Mar 2019 10:51:49 -0700 From: Eric Dumazet <eric.dumazet@...il.com> To: Sabrina Dubroca <sd@...asysnail.net>, 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 On 03/14/2019 10:40 AM, Sabrina Dubroca wrote: > 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. > It might make sense to add a helper using dev_queue_xmit() for tunnel users. Then remove the xmit recursion stuff out of the dev_queue_xmit() Lets make the fast path fast again.
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