netdev - Re: [PATCH net-next 3/3] udp: only use paged allocation with scatter-gather

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Date:   Tue, 15 May 2018 10:14:20 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next 3/3] udp: only use paged allocation with scatter-gather

On Mon, May 14, 2018 at 7:45 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
> On 05/14/2018 04:30 PM, Willem de Bruijn wrote:
>
>> I don't quite follow. The reported crash happens in the protocol layer,
>> because of this check. With pagedlen we have not allocated
>> sufficient space for the skb_put.
>>
>>                 if (!(rt->dst.dev->features&NETIF_F_SG)) {
>>                         unsigned int off;
>>
>>                         off = skb->len;
>>                         if (getfrag(from, skb_put(skb, copy),
>>                                         offset, copy, off, skb) < 0) {
>>                                 __skb_trim(skb, off);
>>                                 err = -EFAULT;
>>                                 goto error;
>>                         }
>>                 } else {
>>                         int i = skb_shinfo(skb)->nr_frags;
>>
>> Are you referring to a separate potential issue in the gso layer?
>> If a bonding device advertises SG, but a slave does not, then
>> skb_segment on the slave should build linear segs? I have not
>> tested that.
>
> Given that the device attribute could change under us, we need to not
> crash, even if initially we thought NETIF_F_SG was available.
>
> Unless you want to hold RTNL in UDP xmit :)
>
> Ideally, GSO should be always on, as we did for TCP.
>
> Otherwise, I can guarantee syzkaller will hit again.

Ah, right. Thanks, Eric!

I'll read that feature bit only once.