netfilter: nft_fib: Mask upper DSCP bits before FIB lookup

As part of its functionality, the nftables FIB expression module
performs a FIB lookup, but unlike other users of the FIB lookup API, it
does so without masking the upper DSCP bits. In particular, this differs
from the equivalent iptables match ("rpfilter") that does mask the upper
DSCP bits before the FIB lookup.

Align the module to other users of the FIB lookup API and mask the upper
DSCP bits using IPTOS_RT_MASK before the lookup.

No regressions in nft_fib.sh:

 # ./nft_fib.sh
 PASS: fib expression did not cause unwanted packet drops
 PASS: fib expression did drop packets for 1.1.1.1
 PASS: fib expression did drop packets for 1c3::c01d
 PASS: fib expression forward check with policy based routing
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/netfilter/nft_fib_ipv4.c

@@ -22,8 +22,6 @@ static __be32 get_saddr(__be32 addr)
 	return addr;
 }
 
-#define DSCP_BITS     0xfc
-
 void nft_fib4_eval_type(const struct nft_expr *expr, struct nft_regs *regs,
 			const struct nft_pktinfo *pkt)
 {
@@ -110,7 +108,7 @@ void nft_fib4_eval(const struct nft_expr *expr, struct nft_regs *regs,
 	if (priv->flags & NFTA_FIB_F_MARK)
 		fl4.flowi4_mark = pkt->skb->mark;
 
-	fl4.flowi4_tos = iph->tos & DSCP_BITS;
+	fl4.flowi4_tos = iph->tos & IPTOS_RT_MASK;
 
 	if (priv->flags & NFTA_FIB_F_DADDR) {
 		fl4.daddr = iph->daddr;