netdev - Re: [PATCH net] skbuff: Fix skb checksum flag on skb pull

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Message-ID: <CALx6S37L-rAT-sL2FHau+bs1gZvh27g3V1G7_Wnm8aZ4vHgVOg@mail.gmail.com>
Date:	Mon, 31 Aug 2015 20:48:57 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Pravin Shelar <pshelar@...ira.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] skbuff: Fix skb checksum flag on skb pull

On Mon, Aug 31, 2015 at 8:25 PM, Pravin Shelar <pshelar@...ira.com> wrote:
> On Mon, Aug 31, 2015 at 4:44 PM, Tom Herbert <tom@...bertland.com> wrote:
>> On Mon, Aug 31, 2015 at 3:55 PM, Pravin B Shelar <pshelar@...ira.com> wrote:
>>> VXLAN device can receive skb with checksum partial. But the checksum
>>> offset could be in outer header which is pulled on receive. Such skb
>>> can cause the panic when checksum is calculated on skb. Following patch
>>> fixes the bug by setting checksum unnecessary while pulling outer header.
>>>
>>> ---8<---
>>> [ 13.800141] RIP: 0010:[<ffffffff81518034>] [<ffffffff81518034>] skb_checksum_help+0x144/0x150
>>> [ 13.800141] RSP: 0000:ffff88011fd83940 EFLAGS: 00010292
>>> [ 13.800141] RAX: 0000000000000042 RBX: ffff880114dd56c0 RCX: ffff8801188d9580
>>> ...
>>> ...
>>> [ 13.852308] Call Trace:
>>> [ 13.852308] <IRQ>
>>> [ 13.852308] [<ffffffffa0164c28>] queue_userspace_packet+0x408/0x470 [openvswitch]
>>> [ 13.852308] [<ffffffffa016614d>] ovs_dp_upcall+0x5d/0x60 [openvswitch]
>>> [ 13.852308] [<ffffffffa0166236>] ovs_dp_process_packet_with_key+0xe6/0x100 [openvswitch]
>>> [ 13.852308] [<ffffffffa016629b>] ovs_dp_process_received_packet+0x4b/0x80 [openvswitch]
>>> [ 13.852308] [<ffffffffa016c51a>] ovs_vport_receive+0x2a/0x30 [openvswitch]
>>> [ 13.852308] [<ffffffffa0171383>] vxlan_rcv+0x53/0x60 [openvswitch]
>>> [ 13.852308] [<ffffffffa01734cb>] vxlan_udp_encap_recv+0x8b/0xf0 [openvswitch]
>>> [ 13.852308] [<ffffffff8157addc>] udp_queue_rcv_skb+0x2dc/0x3b0
>>> [ 13.852308] [<ffffffff8157b56f>] __udp4_lib_rcv+0x1cf/0x6c0
>>> [ 13.852308] [<ffffffff8157ba7a>] udp_rcv+0x1a/0x20
>>> [ 13.852308] [<ffffffff8154fdbd>] ip_local_deliver_finish+0xdd/0x280
>>> [ 13.852308] [<ffffffff81550128>] ip_local_deliver+0x88/0x90
>>> [ 13.852308] [<ffffffff8154fa7d>] ip_rcv_finish+0x10d/0x370
>>> [ 13.852308] [<ffffffff81550365>] ip_rcv+0x235/0x300
>>> [ 13.852308] [<ffffffff8151ba1d>] __netif_receive_skb+0x55d/0x620
>>> [ 13.852308] [<ffffffff8151c360>] netif_receive_skb+0x80/0x90
>>> [ 13.852308] [<ffffffff81459935>] virtnet_poll+0x555/0x6f0
>>> [ 13.852308] [<ffffffff8151cd04>] net_rx_action+0x134/0x290
>>> [ 13.852308] [<ffffffff810683d8>] __do_softirq+0xa8/0x210
>>> [ 13.852308] [<ffffffff8162fe6c>] call_softirq+0x1c/0x30
>>> [ 13.852308] [<ffffffff810161a5>] do_softirq+0x65/0xa0
>>> [ 13.852308] [<ffffffff810687be>] irq_exit+0x8e/0xb0
>>> [ 13.852308] [<ffffffff81630733>] do_IRQ+0x63/0xe0
>>> [ 13.852308] [<ffffffff81625f2e>] common_interrupt+0x6e/0x6e
>>> [ 13.852308] <EOI>
>>> [ 13.852308] [<ffffffff8162dc02>] ? system_call_fastpath+0x16/0x1b
>>>
>>> Reported-by: Anupam Chanda <achanda@...are.com>
>>> Signed-off-by: Pravin B Shelar <pshelar@...ira.com>
>>> ---
>>>  include/linux/skbuff.h |    3 +++
>>>  1 files changed, 3 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
>>> index 9b88536..6238e9f 100644
>>> --- a/include/linux/skbuff.h
>>> +++ b/include/linux/skbuff.h
>>> @@ -2601,6 +2601,9 @@ static inline void skb_postpull_rcsum(struct sk_buff *skb,
>>>  {
>>>         if (skb->ip_summed == CHECKSUM_COMPLETE)
>>>                 skb->csum = csum_sub(skb->csum, csum_partial(start, len, 0));
>>> +       else if (skb->ip_summed == CHECKSUM_PARTIAL &&
>>> +                skb_checksum_start_offset(skb) <= len)
>>> +               skb->ip_summed = CHECKSUM_UNNECESSARY;
>>
>> No, this isn't right. We should never be converting CHECKSUM_PARTIAL
>> into CHECKSUM_UNNECESSARY.
>>
>
> But checksum is valid for inner packet. So I do not see any other
> appropriate checksum flag here. Can you suggest another flag which is
> better suited?

You don't need to do any conversion. skb_checksum_unnecessary checks
CHECKSUM_PARTIAL values. If the checksum offset being checked goes
past CHECKSUM_PARTIAL, then skb_checksum_unnecessary will fail and
checksum complete will be called to get CHECKSUM_COMPLETE. I'm not
sure why openvswitch is even calling skb_checksum_help in the first
place, the stack should be able to handle CHECKSUM_PARTIAL.
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