Re: [PATCH] net: tulip: xircom_cb: drop runt frames before skb copy

On Fri, Jul 31, 2026 at 08:26:29PM +0100, David Laight wrote:
> On Fri, 31 Jul 2026 20:33:34 +0200
> Pablo Vallespín Aranguren <[email protected]> wrote:
> 
> Have you checked that that hardware can actually set a short value?
> I'd expect that packets shorter than 64 bytes (including the crc) are
> dropped as 'runts' and probably don't even use a ring entry.

I haven't verified how the real Xircom firmware handles runt frames (I
don't own this hardware). I tested with Qemu's emulated NIC, the device
itself does validate and reject negative-size values before sending them
to the driver.

> Of course, if you think the device might lie all bets are off.
> (Without an iommu is can write anywhere in host memory...)

I took into account that the hardware could glitch or a misbehaving/fake
card could be used. I modified the emulated NIC to mimic this behaviour,
and given that the driver does not perform a check on its own (to
validate that pkt_len is not negative) it does lead to a kernel panic.

Best,

Pablo

> > Signed-off-by: Pablo Vallespín Aranguren <[email protected]>
> > Assisted-by: gkh_clanker_t1000
> > ---
> >  drivers/net/ethernet/dec/tulip/xircom_cb.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/drivers/net/ethernet/dec/tulip/xircom_cb.c b/drivers/net/ethernet/dec/tulip/xircom_cb.c
> > index e5d2ede13845..62c64da1c8fa 100644
> > --- a/drivers/net/ethernet/dec/tulip/xircom_cb.c
> > +++ b/drivers/net/ethernet/dec/tulip/xircom_cb.c
> > @@ -1110,6 +1110,11 @@ investigate_read_descriptor(struct net_device *dev, struct xircom_private *card,
> >  					/* minus 4, we don't want the CRC */
> >  		struct sk_buff *skb;
> >  
> > +		if (pkt_len < 0) {
> > +			dev->stats.rx_length_errors++;
> > +			goto out;
> > +		}
> > +
> >  		if (pkt_len > 1518) {
> >  			netdev_err(dev, "Packet length %i is bogus\n", pkt_len);
> >  			pkt_len = 1518;
>