On Tue, Sep 10, 2024 at 10:54 PM Willem de Bruijn willemdebruijn.kernel@gmail.com wrote:
Willem de Bruijn wrote:
Jason Wang wrote:
On Tue, Sep 10, 2024 at 8:40 AM Willem de Bruijn willemdebruijn.kernel@gmail.com wrote:
From: Willem de Bruijn willemb@google.com
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:
- 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.
- 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@port70.net/ Fixes: 89add40066f9 ("net: drop bad gso csum_start and offset in virtio_net_hdr") Signed-off-by: Willem de Bruijn willemb@google.com Cc: stable@vger.kernel.net
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.
Or it could be a physical device that can do GRO HW.
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.
Yes.
One missing piece is that TCP GSO will fix up non CHECKSUM_PARTIAL skbs. UDP GSO does not have the same logic.
Thanks