netdev - Re: Possible unsafe usage of skb->cb in virtio-net

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Date:   Fri, 3 Nov 2017 13:57:03 +0900
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Ilya Lesokhin <ilyal@...lanox.com>,
        Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        David Miller <davem@...emloft.net>,
        virtualization@...ts.linux-foundation.org
Subject: Re: Possible unsafe usage of skb->cb in virtio-net

On Thu, Nov 2, 2017 at 10:01 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Thu, Nov 02, 2017 at 11:40:36AM +0000, Ilya Lesokhin wrote:
>> Hi,
>> I've noticed that the virtio-net uses skb->cb.
>>
>> I don't know all the detail by my understanding is it caused problem with the mlx5 driver
>> and was fixed here:
>> https://github.com/torvalds/linux/commit/34802a42b3528b0e18ea4517c8b23e1214a09332
>>
>> Thanks,
>> Ilya
>
> Thanks a lot for the pointer.
>
> I think this was in response to this:
> https://patchwork.ozlabs.org/patch/558324/
>
>> >
>> > +     skb_push(skb, skb->data - skb_data_orig);
>> >       sq->skb[pi] = skb;
>> >
>> >       MLX5E_TX_SKB_CB(skb)->num_wqebbs = DIV_ROUND_UP(ds_cnt,
>>
>> And in the middle of this we have:
>>
>>                 skb_pull_inline(skb, ihs);
>>
>> This is looks illegal.
>>
>> You must not modify the data pointers of any SKB that you receive for
>> sending via ->ndo_start_xmit() unless you know that absolutely you are
>> the one and only reference that exists to that SKB.
>>
>> And exactly for the case you are trying to "fix" here, you do not.  If
>> the SKB is cloned, or has an elevated users count, someone else can be
>> looking at it exactly at the same time you are messing with the data
>> pointers.
>>
>> I bet mlx4 has this bug too.
>>
>> You must fix this properly, by keeping track of an offset or similar
>> internally to your driver, rather than changing the SKB data pointers.
>
> What virtio does is this:
>
>         can_push = vi->any_header_sg &&
>                 !((unsigned long)skb->data & (__alignof__(*hdr) - 1)) &&
>                 !skb_header_cloned(skb) && skb_headroom(skb) >= hdr_len;
>         /* Even if we can, don't push here yet as this would skew
>          * csum_start offset below. */
>         if (can_push)
>                 hdr = (struct virtio_net_hdr_mrg_rxbuf *)(skb->data - hdr_len);
>         else
>                 hdr = skb_vnet_hdr(skb);
>
>
> This doesn't change the data pointers in a cloned skb but it does change the cb.
> Is it true that it's illegal to touch the cb in a cloned skb then?

I don't have all the context for this bug. But in general, clones do not share
the struct sk_buff, which holds the CB. So skb_push and skb_pull_inline
cannot affect the view of other clones. If an skb is shared, that's a different
story.