netdev - Re: [PATCH net] net: Handle negative checksum offset in skb-checksum-help

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Date:	Mon, 21 Sep 2015 10:45:04 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net: Handle negative checksum offset in skb-checksum-help

On Mon, Sep 21, 2015 at 8:47 AM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On 09/20/2015 11:53 PM, Pravin B Shelar wrote:
>>
>> VXLAN device can receive skb with checksum partial. But the checksum
>> offset could be in outer header which is pulled on receive. This results
>> in negative checksum offset for the skb. Such skb can cause the assert
>> failure in skb_checksum_help(). The patch fixes the bug by checking for
>> negative offset in skb_checksum_help().
>>
>> Following is the kernel panic msg from old kernel hitting the bug.
>>
>> ------------[ cut here ]------------
>> kernel BUG at net/core/dev.c:1906!
>> RIP: 0010:[<ffffffff81518034>] skb_checksum_help+0x144/0x150
>> Call Trace:
>> <IRQ>
>> [<ffffffffa0164c28>] queue_userspace_packet+0x408/0x470 [openvswitch]
>> [<ffffffffa016614d>] ovs_dp_upcall+0x5d/0x60 [openvswitch]
>> [<ffffffffa0166236>] ovs_dp_process_packet_with_key+0xe6/0x100
>> [openvswitch]
>> [<ffffffffa016629b>] ovs_dp_process_received_packet+0x4b/0x80
>> [openvswitch]
>> [<ffffffffa016c51a>] ovs_vport_receive+0x2a/0x30 [openvswitch]
>> [<ffffffffa0171383>] vxlan_rcv+0x53/0x60 [openvswitch]
>> [<ffffffffa01734cb>] vxlan_udp_encap_recv+0x8b/0xf0 [openvswitch]
>> [<ffffffff8157addc>] udp_queue_rcv_skb+0x2dc/0x3b0
>> [<ffffffff8157b56f>] __udp4_lib_rcv+0x1cf/0x6c0
>> [<ffffffff8157ba7a>] udp_rcv+0x1a/0x20
>> [<ffffffff8154fdbd>] ip_local_deliver_finish+0xdd/0x280
>> [<ffffffff81550128>] ip_local_deliver+0x88/0x90
>> [<ffffffff8154fa7d>] ip_rcv_finish+0x10d/0x370
>> [<ffffffff81550365>] ip_rcv+0x235/0x300
>> [<ffffffff8151ba1d>] __netif_receive_skb+0x55d/0x620
>> [<ffffffff8151c360>] netif_receive_skb+0x80/0x90
>> [<ffffffff81459935>] virtnet_poll+0x555/0x6f0
>> [<ffffffff8151cd04>] net_rx_action+0x134/0x290
>> [<ffffffff810683d8>] __do_softirq+0xa8/0x210
>> [<ffffffff8162fe6c>] call_softirq+0x1c/0x30
>> [<ffffffff810161a5>] do_softirq+0x65/0xa0
>> [<ffffffff810687be>] irq_exit+0x8e/0xb0
>> [<ffffffff81630733>] do_IRQ+0x63/0xe0
>> [<ffffffff81625f2e>] common_interrupt+0x6e/0x6e
>>
>> Reported-by: Anupam Chanda <achanda@...are.com>
>> Signed-off-by: Pravin B Shelar <pshelar@...ira.com>
>> ---
>>   net/core/dev.c |    4 +++-
>>   1 files changed, 3 insertions(+), 1 deletions(-)
>>
>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index ee0d628..008f1ae 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -2408,6 +2408,9 @@ int skb_checksum_help(struct sk_buff *skb)
>>                 skb_warn_bad_offload(skb);
>>                 return -EINVAL;
>>         }
>> +       offset = skb_checksum_start_offset(skb);
>> +       if (offset < 0)
>> +               goto out_set_summed;
>>         /* Before computing a checksum, we should make sure no frag could
>>          * be modified by an external entity : checksum could be wrong.
>> @@ -2418,7 +2421,6 @@ int skb_checksum_help(struct sk_buff *skb)
>>                         goto out;
>>         }
>>   -     offset = skb_checksum_start_offset(skb);
>>         BUG_ON(offset >= skb_headlen(skb));
>>         csum = skb_checksum(skb, offset, skb->len - offset, 0);
>
>
> It seems like this is just masking an error instead of fixing it. If the
> offload is bad when you are calling this maybe you should be looking at
> instead clearing the flag that is getting you into the state where you are
> triggering a call to this function.
>

This is not error case. This is packet with checksum offset set for
outer tunnel header. After  the outer header is pulled on tunnel decap
the checksum offset becomes negative. Therefore is does not make sense
to calculate checksum again.
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