ipv4: Reject again rules with high DSCP values

Commit 563f8e9 ("ipv4: Stop taking ECN bits into account in
fib4-rules") replaced the validation test on frh->tos. While the new
test is stricter for ECN bits, it doesn't detect the use of high order
DSCP bits. This would be fine if IPv4 could properly handle them. But
currently, most IPv4 lookups are done with the three high DSCP bits
masked. Therefore, using these bits doesn't lead to the expected
result.

Let's reject such configurations again, so that nobody starts to
use and make any assumption about how the stack handles the three high
order DSCP bits in fib4 rules.

Fixes: 563f8e9 ("ipv4: Stop taking ECN bits into account in fib4-rules")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_rules.c

@@ -231,6 +231,11 @@ static int fib4_rule_configure(struct fib_rule *rule, struct sk_buff *skb,
 			       "Invalid dsfield (tos): ECN bits must be 0");
 		goto errout;
 	}
+	/* IPv4 currently doesn't handle high order DSCP bits correctly */
+	if (frh->tos & ~IPTOS_TOS_MASK) {
+		NL_SET_ERR_MSG(extack, "Invalid tos");
+		goto errout;
+	}
 	rule4->dscp = inet_dsfield_to_dscp(frh->tos);
 
 	/* split local/main if they are not already split */