netfilter: nft_exthdr: Fix for unsafe packet data read

While iterating through an SCTP packet's chunks, skb_header_pointer() is
called for the minimum expected chunk header size. If (that part of) the
skbuff is non-linear, the following memcpy() may read data past
temporary buffer '_sch'. Use skb_copy_bits() instead which does the
right thing in this situation.

Fixes: 133dc203 ("netfilter: nft_exthdr: Support SCTP chunks")
Suggested-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_exthdr.c

@@ -327,7 +327,9 @@ static void nft_exthdr_sctp_eval(const struct nft_expr *expr,
 				break;
 
 			dest[priv->len / NFT_REG32_SIZE] = 0;
-			memcpy(dest, (char *)sch + priv->offset, priv->len);
+			if (skb_copy_bits(pkt->skb, offset + priv->offset,
+					  dest, priv->len) < 0)
+				break;
 			return;
 		}
 		offset += SCTP_PAD4(ntohs(sch->length));