Jason Wang wrote: > On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn > <willemdebruijn.kernel@xxxxxxxxx> 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> > > > > --- > > > > 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 > > Not for this patch, but I see this in UDP_L4: > > if (!(hdr->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM)) > return -EINVAL; > > This seems to forbid VIRTIO_NET_HDR_F_DATA_VALID. I wonder what's the > reason for doing this. It tests &, not == ? > > 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 > > > > Acked-by: Jason Wang <jasowang@xxxxxxxxxx> Thanks for reviewing
