Re: [PATCH net-next v2 2/7] um: vector: honor virtio net hdr conversion failures

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Oct 15, 2026 at 02:55:23PM +0200, Johannes Berg wrote:
> On Thu, 2026-10-15 at 08:52 -0400, Michael S. Tsirkin wrote:
> > On Thu, Oct 15, 2026 at 02:47:01PM +0200, Johannes Berg wrote:
> > > On Thu, 2026-10-15 at 08:41 -0400, Michael S. Tsirkin wrote:
> > > > virtio_net_hdr_to_skb() refuses packets whose checksum metadata
> > > > it cannot make sense of, but the vector raw transport ignored
> > > > its return value: rejected packets went on into the stack with
> > > > their metadata unvalidated.
> > > > 
> > > > Propagate the failure.
> > > > 
> > > > Assisted-by: LLM
> > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > ---
> > > >  arch/um/drivers/vector_transports.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> > > > index e1fb2a76fdff..8b7548458663 100644
> > > > --- a/arch/um/drivers/vector_transports.c
> > > > +++ b/arch/um/drivers/vector_transports.c
> > > > @@ -210,7 +210,9 @@ static int raw_verify_header(
> > > >  		return 1;
> > > >  
> > > >  	skb_set_network_header(skb, ETH_HLEN);
> > > > -	virtio_net_hdr_to_skb(skb, vheader, virtio_legacy_is_little_endian());
> > > > +	if (virtio_net_hdr_to_skb(skb, vheader,
> > > > +			virtio_legacy_is_little_endian()) < 0)
> > > > +		return -1;
> > > >  	return 0;
> > > > 
> > > 
> > > Seems it'd be simpler to just
> > > 
> > > 	return virtio_net_hdr_to_skb(...);
> > > 
> > > As far as I can tell, the function only returns -EINVAL or 0 (never a
> > > positive value), and the caller only cares about negative (error) or
> > > positive (no checksum needed), so that'd be equivalent?
> > > 
> > > johannes
> > 
> > if raw_verify_header can safely return any errno - i did not want to
> > audit it.
> 
> You already used an LLM and still didn't want to? ;-)
> 
> I checked briefly (by hand) and it seems fine, it's assigned to the
> function pointer, which only has two callers on the _kern.c side.
> 
> johannes


                               header_check = vp->verify_header(
                                        mmsg_vector->msg_hdr.msg_iov->iov_base,
                                        skb,
                                        vp
                                );
                                if (header_check < 0) {
                                /* Overlay header failed to verify - discard.
                                 * We can actually keep this skb and reuse it,
                                 * but that will make the prep logic too
                                 * complex.
                                 */
                                        dev_kfree_skb_irq(skb);
                                        vp->estats.rx_encaps_errors++;
                                        (*skbuff_vector) = NULL;
                                        mmsg_vector++;
                                        skbuff_vector++;
                                        continue;
                                }
                                if (header_check > 0) {
                                        vp->estats.rx_csum_offload_good++;
                                        skb->ip_summed = CHECKSUM_UNNECESSARY;
                                }



so any negative is fine, but if we return 1 then it will force to CHECKSUM_UNNECESSARY
which I am not sure is what was intended?


-- 
MST