netdev - Re: [PATCH net] packet: in packet_snd start writing at link layer allocation

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Date:   Thu, 24 May 2018 11:17:32 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Tariq Toukan <tariqt@...lanox.com>
Cc:     David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Maor Gottlieb <maorg@...lanox.com>
Subject: Re: [PATCH net] packet: in packet_snd start writing at link layer allocation

On Thu, May 24, 2018 at 11:07 AM, Tariq Toukan <tariqt@...lanox.com> wrote:
>
>
> On 14/05/2018 3:20 AM, David Miller wrote:
>>
>> From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
>> Date: Fri, 11 May 2018 13:24:25 -0400
>>
>>> From: Willem de Bruijn <willemb@...gle.com>
>>>
>>> Packet sockets allow construction of packets shorter than
>>> dev->hard_header_len to accommodate protocols with variable length
>>> link layer headers. These packets are padded to dev->hard_header_len,
>>> because some device drivers interpret that as a minimum packet size.
>>>
>>> packet_snd reserves dev->hard_header_len bytes on allocation.
>>> SOCK_DGRAM sockets call skb_push in dev_hard_header() to ensure that
>>> link layer headers are stored in the reserved range. SOCK_RAW sockets
>>> do the same in tpacket_snd, but not in packet_snd.
>>>
>>> Syzbot was able to send a zero byte packet to a device with massive
>>> 116B link layer header, causing padding to cross over into skb_shinfo.
>>> Fix this by writing from the start of the llheader reserved range also
>>> in the case of packet_snd/SOCK_RAW.
>>>
>>> Update skb_set_network_header to the new offset. This also corrects
>>> it for SOCK_DGRAM, where it incorrectly double counted reserve due to
>>> the skb_push in dev_hard_header.
>>>
>>> Fixes: 9ed988cd5915 ("packet: validate variable length ll headers")
>>> Reported-by: syzbot+71d74a5406d02057d559@...kaller.appspotmail.com
>>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>>
>>
>> Applied and queued up for -stable, thanks Willem.
>>
>
> Hi,
>
> One of our regression tests started failing. Once this patch is reverted,
> test passes.
>
> The tests add flow steering rules in the receiver side and in the sender
> side it send the packet with some RAW socket applications. Then received
> side gets completion with error.
>
> Our verification team compared the packets between the stable and the broken
> version, in the broken version we have some extra bytes at the end of the
> packet.
>
> It looks like some bad push to the SKB, maybe the conditional reserved
> addition should be more strict?
>
> Any idea?

Thanks for reporting, sorry for the breakage.

I think I might. This skb_push moves back the start of skb->data in the
same way that tpacket_snd does. But it does not reduce the length
passed to skb_put, so this might double count hard_header_len.

Let me construct a test.