On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn wrote: > From: Willem de Bruijn <willemb@xxxxxxxxxx> > > The referenced commit drops bad input, but has false positives. > Tighten the check to avoid these. > > The check detects illegal checksum offload requests, which produce > csum_start/csum_off beyond end of packet after segmentation. > > But it is based on two incorrect assumptions: > > 1. virtio_net_hdr_to_skb with VIRTIO_NET_HDR_GSO_TCP[46] implies GSO. > True in callers that inject into the tx path, such as tap. > But false in callers that inject into rx, like virtio-net. > Here, the flags indicate GRO, and CHECKSUM_UNNECESSARY or > CHECKSUM_NONE without VIRTIO_NET_HDR_F_NEEDS_CSUM is normal. > > 2. TSO requires checksum offload, i.e., ip_summed == CHECKSUM_PARTIAL. > False, as tcp[46]_gso_segment will fix up csum_start and offset for > all other ip_summed by calling __tcp_v4_send_check. > > Because of 2, we can limit the scope of the fix to virtio_net_hdr > that do try to set these fields, with a bogus value. > > Link: https://lore.kernel.org/netdev/20240909094527.GA3048202@xxxxxxxxxx/ > Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr") > Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> But I think netdev maintainers ask contributors not to CC stable directly. > --- > > Verified that the syzbot repro is still caught. > > An equivalent alternative would be to move the check for csum_offset > to where the csum_start check is in segmentation: > > - if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb))) > + if (unlikely(skb_checksum_start(skb) != skb_transport_header(skb) || > + skb->csum_offset != offsetof(struct tcphdr, check))) > > Cleaner, but messier stable backport. > > We'll need an equivalent patch to this for VIRTIO_NET_HDR_GSO_UDP_L4. > But that csum_offset test was in a different commit, so different > Fixes tag. > --- > include/linux/virtio_net.h | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h > index 6c395a2600e8d..276ca543ef44d 100644 > --- a/include/linux/virtio_net.h > +++ b/include/linux/virtio_net.h > @@ -173,7 +173,8 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb, > break; > case SKB_GSO_TCPV4: > case SKB_GSO_TCPV6: > - if (skb->csum_offset != offsetof(struct tcphdr, check)) > + if (skb->ip_summed == CHECKSUM_PARTIAL && > + skb->csum_offset != offsetof(struct tcphdr, check)) > return -EINVAL; > break; > } > -- > 2.46.0.598.g6f2099f65c-goog
