Hi all,
Ok, let me prepare a new patch v2, where I'll write a
description/analysis of the issue in the commit message.

On Thu, Feb 22, 2024 at 10:02 PM Michael S. Tsirkin [off-list ref] wrote:

On Mon, Jan 15, 2024 at 05:32:25PM -0500, Michael S. Tsirkin wrote:

quoted

On Mon, Jan 15, 2024 at 09:48:40PM +0200, Andrew Melnychenko wrote:

quoted

When the Qemu launched with vhost but without tap vnet_hdr,
vhost tries to copy vnet_hdr from socket iter with size 0
to the page that may contain some trash.
That trash can be interpreted as unpredictable values for
vnet_hdr.
That leads to dropping some packets and in some cases to
stalling vhost routine when the vhost_net tries to process
packets and fails in a loop.

Qemu options:
  -netdev tap,vhost=on,vnet_hdr=off,...

Signed-off-by: Andrew Melnychenko <redacted>
---
 drivers/vhost/net.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index f2ed7167c848..57411ac2d08b 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c

@@ -735,6 +735,9 @@ static int vhost_net_build_xdp(struct vhost_net_virtqueue *nvq,
    hdr = buf;
    gso = &hdr->gso;

+   if (!sock_hlen)
+           memset(buf, 0, pad);
+
    if ((gso->flags & VIRTIO_NET_HDR_F_NEEDS_CSUM) &&
        vhost16_to_cpu(vq, gso->csum_start) +
        vhost16_to_cpu(vq, gso->csum_offset) + 2 >

Hmm need to analyse it to make sure there are no cases where we leak
some data to guest here in case where sock_hlen is set ...

Could you post this analysis pls?

quoted

quoted

--
2.43.0