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

From: Alexander Duyck <[email-masked]>
Date: Mon, 21 Sep 2015 08:47:06 -0700

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 <[email-masked]>
> Signed-off-by: Pravin B Shelar <[email-masked]>
> ---
>   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.

- Alex