Re: KASAN: use-after-free Read in ipv6_gso_pull_exthdrs

[Willem de Bruijn]

On Fri, Jul 6, 2018 at 1:55 PM syzbot
<syzbot+7b9ed9872dab8c32305d@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> syzbot has found a reproducer for the following crash on:
>
> HEAD commit:    70ba5b6db96f ipv4: Return EINVAL when ping_group_range sys..
> git tree:       net
> console output: https://syzkaller.appspot.com/x/log.txt?x=13cd2970400000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=2ca6c7a31d407f86
> dashboard link: https://syzkaller.appspot.com/bug?extid=7b9ed9872dab8c32305d
> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=15dfb748400000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12a1050c400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+7b9ed9872dab8c32305d@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> IPv6: ADDRCONF(NETDEV_UP): veth0: link is not ready
> IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
> IPv6: ADDRCONF(NETDEV_CHANGE): bond0: link becomes ready
> 8021q: adding VLAN 0 to HW filter on device team0
> ==================================================================
> BUG: KASAN: use-after-free in ipv6_gso_pull_exthdrs+0x57a/0x5f0
> net/ipv6/ip6_offload.c:45
> Read of size 1 at addr ffff8801ce17f3a9 by task syz-executor655/4567

This is an 8 byte packet with a virtio_net_hdr describing it as
VIRTIO_NET_HDR_GSO_TCPV4, but with protocol ETH_P_NSH. That
matches the occurrence of nsh_gso_segment in the stack trace.

This pulls the struct nshhdr of 8B, passing a packet with skb->len 0
to skb_mac_gso_segment. That is the only GRO function that
unconditionally calls _skb_pull without first checking pskb_may_pull.
Adding that check does catch this:

+       if (unlikely(!pskb_may_pull(skb, vlan_depth)))
+               return ERR_PTR(-EINVAL);