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

[Samuel Thibault <samuel.thibault@gnu.org>]

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 <samuel.thibault@ens-lyon.org>
---

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;