ipv4: icmp: fix null-ptr-deref in icmp_build_probe()

ipv6_stub->ipv6_dev_find() may return ERR_PTR(-EAFNOSUPPORT) when the
IPv6 stack is not active (CONFIG_IPV6=m and not loaded), and passing
this error pointer to dev_hold() will cause a kernel crash with
null-ptr-deref.

Instead, silently discard the request. RFC 8335 does not appear to
define a specific response for the case where an IPv6 interface
identifier is syntactically valid but the implementation cannot perform
the lookup at runtime, and silently dropping the request may safer than
misreporting "No Such Interface".

Fixes: d329ea5 ("icmp: add response to RFC 8335 PROBE messages")
Signed-off-by: Yiqi Sun <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: sunichi

net/ipv4/icmp.c

@@ -1346,6 +1346,13 @@ bool icmp_build_probe(struct sk_buff *skb, struct icmphdr *icmphdr)
 			if (iio->ident.addr.ctype3_hdr.addrlen != sizeof(struct in6_addr))
 				goto send_mal_query;
 			dev = ipv6_stub->ipv6_dev_find(net, &iio->ident.addr.ip_addr.ipv6_addr, dev);
+			/*
+			 * If IPv6 identifier lookup is unavailable, silently
+			 * discard the request instead of misreporting NO_IF.
+			 */
+			if (IS_ERR(dev))
+				return false;
+
 			dev_hold(dev);
 			break;
 #endif