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SubjectRe: general protection fault in skb_segment
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

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