ipv4: Fix incorrect source address in Record Route option

The Record Route IP option records the addresses of the routers that
routed the packet. In the case of forwarded packets, the kernel performs
a route lookup via fib_lookup() and fills in the preferred source
address of the matched route.

The lookup is performed with the DS field of the forwarded packet, but
using the RT_TOS() macro which only masks one of the two ECN bits. If
the packet is ECT(0) or CE, the matched route might be different than
the route via which the packet was forwarded as the input path masks
both of the ECN bits, resulting in the wrong address being filled in the
Record Route option.

Fix by masking both of the ECN bits.

Fixes: 8e36360a ("ipv4: Remove route key identity dependencies in ip_rt_get_source().")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20240718123407.434778-1-idosch@nvidia.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/route.c

@@ -1263,7 +1263,7 @@ void ip_rt_get_source(u8 *addr, struct sk_buff *skb, struct rtable *rt)
 		struct flowi4 fl4 = {
 			.daddr = iph->daddr,
 			.saddr = iph->saddr,
-			.flowi4_tos = RT_TOS(iph->tos),
+			.flowi4_tos = iph->tos & IPTOS_RT_MASK,
 			.flowi4_oif = rt->dst.dev->ifindex,
 			.flowi4_iif = skb->dev->ifindex,
 			.flowi4_mark = skb->mark,