Subject: Re: general protection fault in skb_segment

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Sun, 31 Dec 2017 00:25:24 -0200

On Sat, Dec 30, 2017 at 10:52:20PM -0200, Marcelo Ricardo Leitner wrote:
> On Sat, Dec 30, 2017 at 08:42:41AM +0100, Willem de Bruijn wrote:
[...]
> > Somewhat tangential, but any PF_PACKET socket can set this
> > magic gso_size value in its virtio_net_hdr, so if it is assumed to
> > be an SCTP GSO specific option, setting it for a TCP GSO packet
> > may also cause unexpected results.
> 
> It seems virtio_net could use more sanity checks. When PACKET_VNET_HDR
> is used, it will end up calling:
> tpacket_rcv() {
> ...
>         if (do_vnet) {
>                 if (virtio_net_hdr_from_skb(skb, h.raw + macoff -
>                                             sizeof(struct virtio_net_hdr),
>                                             vio_le(), true)) {
>                         spin_lock(&sk->sk_receive_queue.lock);
>                         goto drop_n_account;
>                 }
>         }
> 
> and virtio_net_hdr_from_skb does:
>         if (skb_is_gso(skb)) {
> ...
>                 if (sinfo->gso_type & SKB_GSO_TCPV4)
>                         hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
>                 else if (sinfo->gso_type & SKB_GSO_TCPV6)
>                         hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
>                 else
>                         return -EINVAL;
> 
> Meaning that any gso_type other than TCP would be rejected, but this
> SCTP one got through. Seems the header contains a sctp header, but the
> gso_type set was actually pointing to TCP (otherwise it would have
> been rejected). AFAICT if this packet had an ESP header, for example,
> it could have hit esp4_gso_segment. Can you please confirm this?

I added:
--- a/net/sctp/offload.c
+++ b/net/sctp/offload.c
@@ -44,6 +44,18 @@ static struct sk_buff *sctp_gso_segment(struct sk_buff *skb,
 {
        struct sk_buff *segs = ERR_PTR(-EINVAL);
        struct sctphdr *sh;
+       int fail = 0;
+
+       if (!(skb_shinfo(skb)->gso_type & SKB_GSO_SCTP)) {
+               printk("Bogus gso_type: %x\n", skb_shinfo(skb)->gso_type);
+               fail = 1;
+       }
+       if (skb_shinfo(skb)->gso_size != GSO_BY_FRAGS) {
+               printk("Bogus gso_size: %u\n", skb_shinfo(skb)->gso_size);
+               fail = 1;
+       }
+       if (fail)
+               goto out;
 
        sh = sctp_hdr(skb);
        if (!pskb_may_pull(skb, sizeof(*sh)))

and with the reproducer, got:
[   54.255469] Bogus gso_type: 7
[   54.258801] Bogus gso_size: 63464
[   54.262532] ------------[ cut here ]------------
[   54.267703] syz0: caps=(0x00000800000058c1, 0x0000000000000000) len=32 data_len=0 gso_size=63464 gso_type=7 ip_summed0
[   54.279777] WARNING: CPU: 1 PID: 13005 at /root/linux/net/core/dev.c:2600 skb_warn_bad_offload+0xd6/0xec

gso_type 7 = SKB_GSO_TCPV4 | SKB_GSO_DODGY | SKB_GSO_TCP_ECN
as the warn indicated too.

Once this gets to sctp_gso_segment, it's too late to avoid the
warning. Would be nice if we could somehow filter this earlier in the
process.

  Marcelo