netdev - Re: [PATCH net] net: Add check for csum_start in skb_partial_csum

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Date:   Tue, 11 Apr 2023 22:03:33 +0800
From:   "luwei (O)" <luwei32@...wei.com>
To:     Eric Dumazet <edumazet@...gle.com>
CC:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        <davem@...emloft.net>, <kuba@...nel.org>, <pabeni@...hat.com>,
        <asml.silence@...il.com>, <imagedong@...cent.com>,
        <brouer@...hat.com>, <keescook@...omium.org>, <jbenc@...hat.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net] net: Add check for csum_start in
 skb_partial_csum_set()


在 2023/4/11 4:13 PM, Eric Dumazet 写道:
> On Tue, Apr 11, 2023 at 4:33 AM luwei (O) <luwei32@...wei.com> wrote:
>>
>> 在 2023/4/11 1:30 AM, Willem de Bruijn 写道:
>>
>> Eric Dumazet wrote:
>>
>> On Mon, Apr 10, 2023 at 4:22 AM Lu Wei <luwei32@...wei.com> wrote:
>>
>> If an AF_PACKET socket is used to send packets through a L3 mode ipvlan
>> and a vnet header is set via setsockopt() with the option name of
>> PACKET_VNET_HDR, the value of offset will be nagetive in function
>> skb_checksum_help() and trigger the following warning:
>>
>> WARNING: CPU: 3 PID: 2023 at net/core/dev.c:3262
>> skb_checksum_help+0x2dc/0x390
>> ......
>> Call Trace:
>>   <TASK>
>>   ip_do_fragment+0x63d/0xd00
>>   ip_fragment.constprop.0+0xd2/0x150
>>   __ip_finish_output+0x154/0x1e0
>>   ip_finish_output+0x36/0x1b0
>>   ip_output+0x134/0x240
>>   ip_local_out+0xba/0xe0
>>   ipvlan_process_v4_outbound+0x26d/0x2b0
>>   ipvlan_xmit_mode_l3+0x44b/0x480
>>   ipvlan_queue_xmit+0xd6/0x1d0
>>   ipvlan_start_xmit+0x32/0xa0
>>   dev_hard_start_xmit+0xdf/0x3f0
>>   packet_snd+0xa7d/0x1130
>>   packet_sendmsg+0x7b/0xa0
>>   sock_sendmsg+0x14f/0x160
>>   __sys_sendto+0x209/0x2e0
>>   __x64_sys_sendto+0x7d/0x90
>>
>> The root cause is:
>> 1. skb->csum_start is set in packet_snd() according vnet_hdr:
>>     skb->csum_start = skb_headroom(skb) + (u32)start;
>>
>>     'start' is the offset from skb->data, and mac header has been
>>     set at this moment.
>>
>> 2. when this skb arrives ipvlan_process_outbound(), the mac header
>>     is unset and skb_pull is called to expand the skb headroom.
>>
>> 3. In function skb_checksum_help(), the variable offset is calculated
>>     as:
>>        offset = skb->csum_start - skb_headroom(skb);
>>
>>     since skb headroom is expanded in step2, offset is nagetive, and it
>>     is converted to an unsigned integer when compared with skb_headlen
>>     and trigger the warning.
>>
>> Not sure why it is negative ? This seems like the real problem...
>>
>> csum_start is relative to skb->head, regardless of pull operations.
>>
>> whatever set csum_start to a too small value should be tracked and fixed.
>>
>> Right. The only way I could see it go negative is if something does
>> the equivalent of pskb_expand_head with positive nhead, and without
>> calling skb_headers_offset_update.
>>
>> Perhaps the cause can be found by instrumenting all the above
>> functions in the trace to report skb_headroom and csum_start.
>> And also virtio_net_hdr_to_skb.
>> .
>>
>> Hi, Eric  and Willem,  sorry for not describing this issue clearly enough. Here is the detailed data path:
>>
>> 1.  Users call sendmsg() to send message with a AF_PACKET domain and SOCK_RAW type socket. Since vnet_hdr
>>
>> is set,  csum_start is calculated as:
>>
>>                        skb->csum_start = skb_headroom(skb) + (u32)start;     // see the following code.
>>
>> the varible "start" it passed from user data, in my case it is 5 and skb_headroom is 2, so skb->csum_start is 7.
>>
> I think you are rephrasing, but you did not address my feedback.
>
> Namely, "csum_start < skb->network_header" does not look sensical to me.
>
> csum_start should be related to the transport header, not network header.

     csum_start is calculated in pakcet_snd() as:

                skb->csum_start = skb_headroom(skb) + (u32)start;

    the varible "start" it passed from user data via vnet_hdr as follows:

     packet_snd()
     ...	
	if (po->has_vnet_hdr) {
		err = packet_snd_vnet_parse(msg, &len, &vnet_hdr);   // get vnet_hdr which includes start
		if (err)
		    goto out_unlock;
		has_vnet_hdr = true;
	}
     ...

   csum_start should be at the transport header but users may pass an incorrect value.

>
> If you fix a bug, please fix it completely, instead of leaving room
> for future syzbot reports.
>
> Also, your reference to ipvlan pulling a mac header is irrelevant to
> this bug, and adds confusion.
>
> That is  because csum_start is relative to skb->head, not skb->data.
> So ipvlan business does not change csum_start or skb->head.
> .

-- 
Best Regards,
Lu Wei