Willem de Bruijn wrote: > 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 == ? Oh you mean as alternative, for receive of GRO from hypervisor. Yes, fair point. Then we also trust a privileged process over tun, like syzkaller. When it comes to checksums, I suppose that is fine: it cannot harm kernel integrity. One missing piece is that TCP GSO will fix up non CHECKSUM_PARTIAL skbs. UDP GSO does not have the same logic.
