netdev - Re: [REGRESSION][BISECTED] virtio_net CSUM broken with Cloud Hypervisor

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <87v7jwwqxw.fsf@alyssa.is>
Date: Thu, 30 Oct 2025 12:48:43 +0100
From: Alyssa Ross <hi@...ssa.is>
To: Jason Wang <jasowang@...hat.com>
Cc: Paolo Abeni <pabeni@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
 netdev@...r.kernel.org, virtualization@...ts.linux.dev,
 regressions@...ts.linux.dev
Subject: Re: [REGRESSION][BISECTED] virtio_net CSUM broken with Cloud
 Hypervisor

Jason Wang <jasowang@...hat.com> writes:

> On Thu, Oct 30, 2025 at 4:03 AM Alyssa Ross <hi@...ssa.is> wrote:
>>
>> Since 56a06bd40fab ("virtio_net: enable gso over UDP tunnel support."),
>> networking in Cloud Hypervisor is no longer working for me.
>>
>> I've narrowed down the problem to here:
>>
>> > @@ -2555,14 +2567,21 @@ static void virtnet_receive_done(struct virtnet_info *vi, struct receive_queue *
>> >       if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
>> >               virtio_skb_set_hash(&hdr->hash_v1_hdr, skb);
>> >
>> > -     if (flags & VIRTIO_NET_HDR_F_DATA_VALID)
>> > -             skb->ip_summed = CHECKSUM_UNNECESSARY;
>> > +     hdr->hdr.flags = flags;
>>
>> It looks like this was added because virtio_net_handle_csum_offload()
>> looks at the flags from the hdr it's given, rather than having it passed
>> separately, but it appears something later on relies on the previous
>> value of hdr->hdr.flags.
>>
>> From my tracing, hdr->hdr.flags is set to either 0 or
>> VIRTIO_NET_HDR_F_NEEDS_CSUM before this assignment, and flags is always
>> 0, so in some cases VIRTIO_NET_HDR_F_NEEDS_CSUM now ends up being unset.
>
> Are you using XDP, if not there should be no change.
>
>>
>> > +     if (virtio_net_handle_csum_offload(skb, &hdr->hdr, vi->rx_tnl_csum)) {
>> > +             net_warn_ratelimited("%s: bad csum: flags: %x, gso_type: %x rx_tnl_csum %d\n",
>> > +                                  dev->name, hdr->hdr.flags,
>> > +                                  hdr->hdr.gso_type, vi->rx_tnl_csum);
>> > +             goto frame_err;
>> > +     }
>>
>> If I change it to save the previous value of hdr->hdr.flags, and restore
>> it again here, everything works again.
>>
>> Disabling offload_csum in Cloud Hypervisor is a usable workaround,
>> because then hdr->hdr.flags is always 0 to begin with anyway.
>>
>> #regzbot introduced: 56a06bd40fab
>
> Is mergeable rx buffer enabled, if not, I wonder if this can be fixed by:
>
> https://marc.info/?l=linux-netdev&m=176170721926346&w=2

That patch does fix it, thank you!

#regzbot fix: virtio-net: fix incorrect flags recording in big mode

Download attachment "signature.asc" of type "application/pgp-signature" (228 bytes)