netfilter: nft_meta: fix wrong value dereference in nft_meta_set_eval

In the nft_meta_set_eval, nftrace value is dereferenced as u32 from sreg.
But correct type is u8. so that sometimes incorrect value is dereferenced.

Steps to reproduce:

   %nft add table ip filter
   %nft add chain ip filter input { type filter hook input priority 4\; }
   %nft add rule ip filter input nftrace set 0
   %nft monitor

Sometimes, we can see trace messages.

   trace id 16767227 ip filter input packet: iif "enp2s0"
   ether saddr xx:xx:xx:xx:xx:xx ether daddr xx:xx:xx:xx:xx:xx
   ip saddr 192.168.0.1 ip daddr 255.255.255.255 ip dscp cs0
   ip ecn not-ect ip
   trace id 16767227 ip filter input rule nftrace set 0 (verdict continue)
   trace id 16767227 ip filter input verdict continue
   trace id 16767227 ip filter input

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_meta.c

@@ -234,7 +234,7 @@ void nft_meta_set_eval(const struct nft_expr *expr,
 	struct sk_buff *skb = pkt->skb;
 	u32 *sreg = &regs->data[meta->sreg];
 	u32 value = *sreg;
-	u8 pkt_type;
+	u8 value8;
 
 	switch (meta->key) {
 	case NFT_META_MARK:
@@ -244,15 +244,17 @@ void nft_meta_set_eval(const struct nft_expr *expr,
 		skb->priority = value;
 		break;
 	case NFT_META_PKTTYPE:
-		pkt_type = nft_reg_load8(sreg);
+		value8 = nft_reg_load8(sreg);
 
-		if (skb->pkt_type != pkt_type &&
-		    skb_pkt_type_ok(pkt_type) &&
+		if (skb->pkt_type != value8 &&
+		    skb_pkt_type_ok(value8) &&
 		    skb_pkt_type_ok(skb->pkt_type))
-			skb->pkt_type = pkt_type;
+			skb->pkt_type = value8;
 		break;
 	case NFT_META_NFTRACE:
-		skb->nf_trace = !!value;
+		value8 = nft_reg_load8(sreg);
+
+		skb->nf_trace = !!value8;
 		break;
 	default:
 		WARN_ON(1);