Re: [PATCH net] net: Add check for csum_start in skb_partial_csum_set()

[Eric Dumazet]

On Tue, Apr 11, 2023 at 4:33 AM luwei (O) <luwei32@xxxxxxxxxx> 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@xxxxxxxxxx> 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.

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.