netfilter: nf_tables: fix 'exist' matching on bigendian arches

[ Upstream commit 63331e37fb227e796894b31d713697612c8dee7f ]

Maze reports "tcp option fastopen exists" fails to match on
OpenWrt 22.03.5, r20134-5f15225c1e (5.10.176) router.

"tcp option fastopen exists" translates to:
inet
  [ exthdr load tcpopt 1b @ 34 + 0 present => reg 1 ]
  [ cmp eq reg 1 0x00000001 ]

.. but existing nft userspace generates a 1-byte compare.

On LSB (x86), "*reg32 = 1" is identical to nft_reg_store8(reg32, 1), but
not on MSB, which will place the 1 last. IOW, on bigendian aches the cmp8
is awalys false.

Make sure we store this in a consistent fashion, so existing userspace
will also work on MSB (bigendian).

Regardless of this patch we can also change nft userspace to generate
'reg32 == 0' and 'reg32 != 0' instead of u8 == 0 // u8 == 1 when
adding 'option x missing/exists' expressions as well.

Fixes: 3c1fece881 ("netfilter: nft_exthdr: Allow checking TCP option presence, too")
Fixes: b9f9a485fb ("netfilter: nft_exthdr: add boolean DCCP option matching")
Fixes: 055c4b34b9 ("netfilter: nft_fib: Support existence check")
Reported-by: Maciej Żenczykowski <zenczykowski@gmail.com>
Closes: https://lore.kernel.org/netfilter-devel/CAHo-OozyEqHUjL2-ntATzeZOiuftLWZ_HU6TOM_js4qLfDEAJg@mail.gmail.com/
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/netfilter/nft_exthdr.c

@@ -214,7 +214,7 @@ static void nft_exthdr_tcp_eval(const struct nft_expr *expr,
 
 		offset = i + priv->offset;
 		if (priv->flags & NFT_EXTHDR_F_PRESENT) {
-			*dest = 1;
+			nft_reg_store8(dest, 1);
 		} else {
 			if (priv->len % NFT_REG32_SIZE)
 				dest[priv->len / NFT_REG32_SIZE] = 0;

net/netfilter/nft_fib.c

@@ -140,11 +140,15 @@ void nft_fib_store_result(void *reg, const struct nft_fib *priv,
 	switch (priv->result) {
 	case NFT_FIB_RESULT_OIF:
 		index = dev ? dev->ifindex : 0;
-		*dreg = (priv->flags & NFTA_FIB_F_PRESENT) ? !!index : index;
+		if (priv->flags & NFTA_FIB_F_PRESENT)
+			nft_reg_store8(dreg, !!index);
+		else
+			*dreg = index;
+
 		break;
 	case NFT_FIB_RESULT_OIFNAME:
 		if (priv->flags & NFTA_FIB_F_PRESENT)
-			*dreg = !!dev;
+			nft_reg_store8(dreg, !!dev);
 		else
 			strncpy(reg, dev ? dev->name : "", IFNAMSIZ);
 		break;