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

[Willem de Bruijn]

On Fri, Jul 6, 2018 at 6:16 PM Willem de Bruijn
<willemdebruijn.ker...@gmail.com> wrote:
>
> On Fri, Jul 6, 2018 at 1:55 PM syzbot
> <syzbot+7b9ed9872dab8c323...@syzkaller.appspotmail.com> 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+7b9ed9872dab8c323...@syzkaller.appspotmail.com
> >
> > 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);

vlan_depth is 65528 when entering skb_mac_gso_segment due to
overflow of skb->mac_len in nsh_gso_segment. For this packet, the
outer mac len is zero, so skb->data == skb_mac_header(skb). Then

    skb_reset_network_header(skb);
    [...]
    __skb_pull(skb, nsh_len);
    skb_reset_mac_header(skb);    // now mac hdr starts nsh_len == 8B
after net hdr
    skb_reset_mac_len(skb);          // mac len = net hdr - mac hdr ==
(u16) -8 == 65528
    [..]
    skb_mac_gso_segment(skb, ..)

Setting skb->mac_len to 0, similar to mpls_gs_segment,
is sufficient if the encapsulated packet is not ETH_P_TEB.

If the packet is encapsulated at L2, __skb_pull(skb, vlan_depth)
has to pull the inner mac header before passing to l3 handlers like
inet_gso_segment.

If that header includes VLAN tags, skb_network_protocol will
parse then and update the mac length in vlan_depth. So
hardcoding to ETH_HLEN should be fine:

@@ -104,7 +95,7 @@ static struct sk_buff *nsh_gso_segment(struct sk_buff *skb,
        __skb_pull(skb, nsh_len);

        skb_reset_mac_header(skb);
-       skb_reset_mac_len(skb);
+       skb->mac_len = proto == ETH_P_TEB ? ETH_HLEN : 0;
        skb->protocol = proto;

        features &= NETIF_F_SG;