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

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Date:   Mon, 14 May 2018 16:45:46 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        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 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.