On Mon, Sep 09, 2024 at 08:38:52PM -0400, Willem de Bruijn 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
Acked-by: Michael S. Tsirkin mst@redhat.com
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