Subject: Re: [PATCH] vsock/virtio: fix vsockmon info leak in non-linear tap copy

From: Luigi Leonardi <[email-masked]>
Date: Thu, 30 Apr 2026 15:04:37 +0200

On Thu, Apr 30, 2026 at 03:11:10PM +0800, Yiqi Sun wrote:
>vsockmon mirrors packets through virtio_transport_build_skb(), which
>builds a new skb and copies the payload into it. For non-linear skbs,
>this goes through virtio_transport_copy_nonlinear_skb().
>
>Helper manually initializes a iov_iter, but leaves iov_iter.count unset.
>As a result, skb_copy_datagram_iter() sees zero writable bytes
>in the destination iterator and copies no payload data.
>
>This becomes an info leak because virtio_transport_build_skb() has
>already reserved payload_len bytes in the new skb with skb_put(). The
>skb is then returned to the tap path with that payload area still
>uninitialized, so userspace reading from a vsockmon device can observe
>heap contents and potentially kernel address.
>
>Fix it by initializing iov_iter.count to the number of bytes to copy.
>
>Fixes: 4b0bf10eb077 ("vsock/virtio: non-linear skb handling for tap")
>Signed-off-by: Yiqi Sun <[email-masked]>
>---
> net/vmw_vsock/virtio_transport_common.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/net/vmw_vsock/virtio_transport_common.c b/net/vmw_vsock/virtio_transport_common.c
>index 416d533f493d..6b26ee57ccab 100644
>--- a/net/vmw_vsock/virtio_transport_common.c
>+++ b/net/vmw_vsock/virtio_transport_common.c
>@@ -152,7 +152,7 @@ static void virtio_transport_copy_nonlinear_skb(const struct sk_buff *skb,
> 	iov_iter.nr_segs = 1;
>
> 	to_copy = min_t(size_t, len, skb->len);
>-
>+	iov_iter.count = to_copy;
> 	skb_copy_datagram_iter(skb, VIRTIO_VSOCK_SKB_CB(skb)->offset,
> 			       &iov_iter, to_copy);
> }
>-- 
>2.34.1
>

Thanks for the fix!

Tested using vsock_loopback sending zero-copy packets. Payload is always
zero before the fix.

Reviewed-by: Luigi Leonardi <[email-masked]>