netdev - Re: [PATCH net v2 2/2] tun: always set skb->dev to tun->dev

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Date:   Mon, 7 Jan 2019 17:30:10 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net v2 2/2] tun: always set skb->dev to tun->dev

On Mon, Jan 7, 2019 at 4:41 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> While debugging previous issue I noticed that commit 90e33d459407 ("tun:
> enable napi_gro_frags() for TUN/TAP driver") started conditionally
> (!frags) calling eth_type_trans(skb, tun->dev) for IFF_TAP case. Since
> eth_type_trans sets skb->dev, some skbs can now have NULL skb->dev.
> Fix that by always setting skb->dev unconditionally.
>
> The syzbot fails with the following trace:
> WARNING: CPU: 0 PID: 11136 at net/core/flow_dissector.c:764
>  skb_flow_dissect_flow_keys_basic include/linux/skbuff.h:1240 [inline]
>  skb_probe_transport_header include/linux/skbuff.h:2403 [inline]
>  tun_get_user+0x2d4a/0x4250 drivers/net/tun.c:1906
>  tun_chr_write_iter+0xb9/0x160 drivers/net/tun.c:1993
>  call_write_iter include/linux/fs.h:1808 [inline]
>  new_sync_write fs/read_write.c:474 [inline]
>
> But I don't think there is an actual issue since we exercise flow
> dissector via eth_get_headlen which doesn't use skb (and hence BPF flow
> dissector).

Do you mean skb_probe_transport_header?

if frags, tun_napi_alloc_frags will return napi->skb, which has
skb->dev set by napi_reuse_skb.

I don't think this is needed.