netfilter: nft_payload: cancel register tracking after payload update

The payload expression might mangle the packet, cancel register tracking
since any payload data in the registers is stale.

Finer grain register tracking cancellation by inspecting the payload
base, offset and length on the register is also possible.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_payload.c

@@ -798,12 +798,33 @@ static int nft_payload_set_dump(struct sk_buff *skb, const struct nft_expr *expr
 	return -1;
 }
 
+static bool nft_payload_set_reduce(struct nft_regs_track *track,
+				   const struct nft_expr *expr)
+{
+	int i;
+
+	for (i = 0; i < NFT_REG32_NUM; i++) {
+		if (!track->regs[i].selector)
+			continue;
+
+		if (track->regs[i].selector->ops != &nft_payload_ops &&
+		    track->regs[i].selector->ops != &nft_payload_fast_ops)
+			continue;
+
+		track->regs[i].selector = NULL;
+		track->regs[i].bitwise = NULL;
+	}
+
+	return false;
+}
+
 static const struct nft_expr_ops nft_payload_set_ops = {
 	.type		= &nft_payload_type,
 	.size		= NFT_EXPR_SIZE(sizeof(struct nft_payload_set)),
 	.eval		= nft_payload_set_eval,
 	.init		= nft_payload_set_init,
 	.dump		= nft_payload_set_dump,
+	.reduce		= nft_payload_set_reduce,
 };
 
 static const struct nft_expr_ops *