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[80.230.68.31]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47aa039b0cesm44408115f8f.22.2026.07.08.09.50.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 08 Jul 2026 09:50:08 -0700 (PDT) Date: Wed, 8 Jul 2026 12:50:06 -0400 From: "Michael S. Tsirkin" To: enrico.zanda@arm.com Cc: jasowangio@gmail.com, virtualization@lists.linux.dev, netdev@vger.kernel.org, kuba@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, eperezma@redhat.com, nd@arm.com Subject: Re: [PATCH net] vhost-net: fix TX stall when vhost owns virtio-net header Message-ID: <20260708124901-mutt-send-email-mst@kernel.org> References: <20260708152242.2268848-1-enrico.zanda@arm.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260708152242.2268848-1-enrico.zanda@arm.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: pVa2JNqEe0_Oz8xgrpO3pVbI4zDN_CFbh81OZJ6bvSQ_1783529410 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Wed, Jul 08, 2026 at 04:22:42PM +0100, enrico.zanda@arm.com wrote: > From: Enrico Zanda > > When vhost owns the virtio-net header, i.e. when > VHOST_NET_F_VIRTIO_NET_HDR is negotiated, sock_hlen is 0, > meaning that no header will be forwarded to the TAP device. > > In the current vhost_net_build_xdp() implementation, > when sock_hlen == 0, the gso pointer can point at the start of the > Ethernet frame instead of a virtio-net header. > This results in a wrong interpretation of the destination MAC address > bytes as struct virtio_net_hdr fields. > > This can, for some MAC addresses, trigger -EINVAL and return early > before the TX descriptor is completed, which can stall vhost-net TX. > > Before 97b2409f28e0, the gso pointer was set to the zeroed padding area, > using it as a synthetic virtio-net header. Restore that behavior. > > Fixes: 97b2409f28e0 ("vhost-net: reduce one userspace copy when building XDP buff") > Signed-off-by: Enrico Zanda The fix looks good: Acked-by: Michael S. Tsirkin Sashiko thinks there's something something security here, but I think it is misguided. It's just guest hurting itself. driver breaks the device it gets to keep both pieces. > --- > drivers/vhost/net.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index 77b59f49bddb..3e72b9c6af0c 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -731,10 +731,12 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq, > goto err; > } > > - gso = buf + pad - sock_hlen; > - > - if (!sock_hlen) > + if (!sock_hlen) { > memset(buf, 0, pad); > + gso = buf; > + } else { > + gso = buf + pad - sock_hlen; > + } > > if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) && > vhost16_to_cpu(vq, gso->csum_start) + > -- > 2.43.0