slirp: Don't crash on packets from 0.0.0.0/8.

LWIP can generate packets with a source of 0.0.0.0, which triggers an
assertion failure in arp_table_add().  Instead of crashing, simply return
to avoid adding an invalid ARP table entry.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
---
 slirp/arp_table.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

On 2012-11-12 01:59, Nickolai Zeldovich wrote:
> LWIP can generate packets with a source of 0.0.0.0, which triggers an
> assertion failure in arp_table_add().  Instead of crashing, simply return
> to avoid adding an invalid ARP table entry.

I would prefer to filter out such invalid packets at a different level.
Did you analyzed which path it takes through the stack?

> 
> Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
> ---
>  slirp/arp_table.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/slirp/arp_table.c b/slirp/arp_table.c
> index 5d7b8ac..3318ce9 100644
> --- a/slirp/arp_table.c
> +++ b/slirp/arp_table.c
> @@ -38,7 +38,8 @@ void arp_table_add(Slirp *slirp, uint32_t ip_addr, uint8_t ethaddr[ETH_ALEN])
>                  ethaddr[3], ethaddr[4], ethaddr[5]));
>  
>      /* Check 0.0.0.0/8 invalid source-only addresses */
> -    assert((ip_addr & htonl(~(0xf << 28))) != 0);
> +    if ((ip_addr & htonl(~(0xf << 28))) == 0)
> +        return;

Please follow our coding style. There is also checkpatch.pl to help you.

>  
>      if (ip_addr == 0xffffffff || ip_addr == broadcast_addr) {
>          /* Do not register broadcast addresses */
> 

Jan

On Mon, Nov 12, 2012 at 4:37 AM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> On 2012-11-12 01:59, Nickolai Zeldovich wrote:
>> LWIP can generate packets with a source of 0.0.0.0, which triggers an
>> assertion failure in arp_table_add().  Instead of crashing, simply return
>> to avoid adding an invalid ARP table entry.
>
> I would prefer to filter out such invalid packets at a different level.
> Did you analyzed which path it takes through the stack?

The particular packet that crashed qemu for me was a gratuitous ARP,
though it looks like all three calls to arp_table_add() in arp_input()
can trigger this.

Popping up one level, I'm not sure why arp_table_add() and
arp_table_search() need a special case for 0.0.0.0/8 in the first
place.  I couldn't find any other code that assumes the ARP table
cannot contain 0.0.0.0/8 entries.  Would anything break if the check
for 0.0.0.0/8 was removed from arp_table_add() and arp_table_search()
altogether?

Nickolai.

On 2012-11-12 15:41, Nickolai Zeldovich wrote:
> On Mon, Nov 12, 2012 at 4:37 AM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
>> On 2012-11-12 01:59, Nickolai Zeldovich wrote:
>>> LWIP can generate packets with a source of 0.0.0.0, which triggers an
>>> assertion failure in arp_table_add().  Instead of crashing, simply return
>>> to avoid adding an invalid ARP table entry.
>>
>> I would prefer to filter out such invalid packets at a different level.
>> Did you analyzed which path it takes through the stack?
> 
> The particular packet that crashed qemu for me was a gratuitous ARP,
> though it looks like all three calls to arp_table_add() in arp_input()
> can trigger this.
> 
> Popping up one level, I'm not sure why arp_table_add() and
> arp_table_search() need a special case for 0.0.0.0/8 in the first
> place.  I couldn't find any other code that assumes the ARP table
> cannot contain 0.0.0.0/8 entries.  Would anything break if the check
> for 0.0.0.0/8 was removed from arp_table_add() and arp_table_search()
> altogether?

0.0.0.0/8 are source-only, invalid as destination. So they have no place
in the ARP table.

OK, let's follow your path and filter them in arp_table_add. Just add
the missing braces and resend.

Jan