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

[Jason Wang <jasowang@redhat.com>]

On 2020/11/30 下午8:11, Mauro Matteo Cascella wrote:
> 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.


I think we can go with this patch first and tweak on top consider it's 
near the release. So:

Acked-by: Jason Wang <jasowang@redhat.com>

Peter, do you want to merge this patch?

Thanks


>
>> -- >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,