Re: [Qemu-devel] [PATCHv2, DoS] slirp (arp): do not special-case bogus IP addresses

[Edgar E. Iglesias]

On Wed, May 14, 2014 at 03:13:09AM +0200, Samuel Thibault wrote:
> Do not special-case addresses with zero host part, as we do not
> necessarily know how big it is, and the guest can fake them anyway.
> Silently avoid having 0.0.0.0 as a destination, however.
> 
> Signed-off-by: Samuel Thibault <address@hidden>

Reviewed-by: Edgar E. Iglesias <address@hidden>

> ---
> 
> This is particularly bad actually, one can for instance simply do this
> inside a Linux guest
> 
> ip addr add 192.0.0.0/1 dev eth0
> 
> and crash qemu (thus a DoS) by just emitting a packet (thus from
> 192.0.0.0), getting:
> 
> qemu-system-x86_64: /usr/src/qemu/slirp/arp_table.c:77: arp_table_search: 
> Assertion `(ip_addr & __bswap_32 (~(0xfU << 28))) != 0' failed.
> 
> so it should probably go to all stable maintained versions.
> 
> diff --git a/slirp/arp_table.c b/slirp/arp_table.c
> index ecdb0ba..bcaeb44 100644
> --- a/slirp/arp_table.c
> +++ b/slirp/arp_table.c
> @@ -37,12 +37,7 @@ void arp_table_add(Slirp *slirp, uint32_t ip_addr, uint8_t 
> ethaddr[ETH_ALEN])
>                  ethaddr[0], ethaddr[1], ethaddr[2],
>                  ethaddr[3], ethaddr[4], ethaddr[5]));
>  
> -    /* Check 0.0.0.0/8 invalid source-only addresses */
> -    if ((ip_addr & htonl(~(0xfU << 28))) == 0) {
> -        return;
> -    }
> -
> -    if (ip_addr == 0xffffffff || ip_addr == broadcast_addr) {
> +    if (ip_addr == 0 || ip_addr == 0xffffffff || ip_addr == broadcast_addr) {
>          /* Do not register broadcast addresses */
>          return;
>      }
> @@ -73,9 +68,6 @@ bool arp_table_search(Slirp *slirp, uint32_t ip_addr,
>      DEBUG_CALL("arp_table_search");
>      DEBUG_ARG("ip = 0x%x", ip_addr);
>  
> -    /* Check 0.0.0.0/8 invalid source-only addresses */
> -    assert((ip_addr & htonl(~(0xfU << 28))) != 0);
> -
>      /* If broadcast address */
>      if (ip_addr == 0xffffffff || ip_addr == broadcast_addr) {
>          /* return Ethernet broadcast address */
> diff --git a/slirp/slirp.c b/slirp/slirp.c
> index 3fb48a4..00f4eb5 100644
> --- a/slirp/slirp.c
> +++ b/slirp/slirp.c
> @@ -778,6 +778,11 @@ int if_encap(Slirp *slirp, struct mbuf *ifm)
>          return 1;
>      }
>  
> +    if (iph->ip_dst.s_addr == 0) {
> +        /* 0.0.0.0 can not be a destination address, something went wrong,
> +         * avoid making it worse */
> +        return 1;
> +    }
>      if (!arp_table_search(slirp, iph->ip_dst.s_addr, ethaddr)) {
>          uint8_t arp_req[ETH_HLEN + sizeof(struct arphdr)];
>          struct ethhdr *reh = (struct ethhdr *)arp_req;
>