Re: [PATCH] hw/net/dp8393x: fix integer underflow in dp8393x_do_transmit_packets()

[Mauro Matteo Cascella <mcascell@redhat.com>]

On Mon, Nov 30, 2020 at 11:44 AM Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
>
> +Laurent/Finn
>
> On 11/24/20 10:24 AM, Mauro Matteo Cascella wrote:
> > An integer underflow could occur during packet transmission due to 'tx_len' not
> > being updated if SONIC_TFC register is set to zero. Check for negative 'tx_len'
> > when removing existing FCS.
> >
> > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1899722
> > Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
> > Reported-by: Gaoning Pan <pgn@zju.edu.cn>
> > ---
> >  hw/net/dp8393x.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/hw/net/dp8393x.c b/hw/net/dp8393x.c
> > index 674b04b354..205c0decc5 100644
> > --- a/hw/net/dp8393x.c
> > +++ b/hw/net/dp8393x.c
> > @@ -495,6 +495,10 @@ static void dp8393x_do_transmit_packets(dp8393xState *s)
> >          } else {
> >              /* Remove existing FCS */
> >              tx_len -= 4;
> > +            if (tx_len < 0) {
> > +                SONIC_ERROR("tx_len is %d\n", tx_len);
> > +                break;
> > +            }
> >          }
> >
> >          if (s->regs[SONIC_RCR] & (SONIC_RCR_LB1 | SONIC_RCR_LB0)) {
> >
>
> Doesn't it make more sense to check 'tx_len >= 4'
> and skip tx/rx when 'tx_len == 0'?
>

Yes, it makes sense. I thought that skipping tx/rx in case of null
'tx_len' could lead to potential inconsistencies when writing the
status or reading the footer of the packet. but I'm not really sure. I
can send a new version of the patch if needed, otherwise feel free to
apply your changes. Thank you.

> -- >8 --
> @@ -488,25 +488,29 @@ static void
> dp8393x_do_transmit_packets(dp8393xState *s)
>              }
>          }
>
> -        /* Handle Ethernet checksum */
> -        if (!(s->regs[SONIC_TCR] & SONIC_TCR_CRCI)) {
> -            /* Don't append FCS there, to look like slirp packets
> -             * which don't have one */
> -        } else {
> -            /* Remove existing FCS */
> -            tx_len -= 4;
> +        if (tx_len >= 4) {
> +            /* Handle Ethernet checksum */
> +            if (!(s->regs[SONIC_TCR] & SONIC_TCR_CRCI)) {
> +                /* Don't append FCS there, to look like slirp packets
> +                 * which don't have one */
> +            } else {
> +                /* Remove existing FCS */
> +                tx_len -= 4;
> +            }
>          }
>
> -        if (s->regs[SONIC_RCR] & (SONIC_RCR_LB1 | SONIC_RCR_LB0)) {
> -            /* Loopback */
> -            s->regs[SONIC_TCR] |= SONIC_TCR_CRSL;
> -            if (nc->info->can_receive(nc)) {
> -                s->loopback_packet = 1;
> -                nc->info->receive(nc, s->tx_buffer, tx_len);
> +        if (tx_len > 0) {
> +            if (s->regs[SONIC_RCR] & (SONIC_RCR_LB1 | SONIC_RCR_LB0)) {
> +                /* Loopback */
> +                s->regs[SONIC_TCR] |= SONIC_TCR_CRSL;
> +                if (nc->info->can_receive(nc)) {
> +                    s->loopback_packet = 1;
> +                    nc->info->receive(nc, s->tx_buffer, tx_len);
> +                }
> +            } else {
> +                /* Transmit packet */
> +                qemu_send_packet(nc, s->tx_buffer, tx_len);
>              }
> -        } else {
> -            /* Transmit packet */
> -            qemu_send_packet(nc, s->tx_buffer, tx_len);
>          }
>          s->regs[SONIC_TCR] |= SONIC_TCR_PTX;
>
> ---
>

Regards,
-- 
Mauro Matteo Cascella
Red Hat Product Security
PGP-Key ID: BB3410B0