filter: add SKF_AD_NLATTR_NEST to look for nested attributes

SKF_AD_NLATTR allows us to find the first matching attribute in a
stream of netlink attributes from one offset to the end of the
netlink message. This is not suitable to look for a specific
matching inside a set of nested attributes.

For example, in ctnetlink messages, if we look for the CTA_V6_SRC
attribute in a message that talks about an IPv4 connection,
SKF_AD_NLATTR returns the offset of CTA_STATUS which has the same
value of CTA_V6_SRC but outside the nest. To differenciate
CTA_STATUS and CTA_V6_SRC, we would have to make assumptions on the
size of the attribute and the usual offset, resulting in horrible
BSF code.

This patch adds SKF_AD_NLATTR_NEST, which is a variant of
SKF_AD_NLATTR, that looks for an attribute inside the limits of
a nested attributes, but not further.

This patch validates that we have enough room to look for the
nested attributes - based on a suggestion from Patrick McHardy.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -122,7 +122,8 @@ struct sock_fprog	/* Required for SO_ATTACH_FILTER. */
 #define SKF_AD_PKTTYPE 	4
 #define SKF_AD_IFINDEX 	8
 #define SKF_AD_NLATTR	12
-#define SKF_AD_MAX 	16
+#define SKF_AD_NLATTR_NEST	16
+#define SKF_AD_MAX	20
 #define SKF_NET_OFF   (-0x100000)
 #define SKF_LL_OFF    (-0x200000)
 

net/core/filter.c

@@ -319,6 +319,25 @@ load_b:
 				A = 0;
 			continue;
 		}
+		case SKF_AD_NLATTR_NEST: {
+			struct nlattr *nla;
+
+			if (skb_is_nonlinear(skb))
+				return 0;
+			if (A > skb->len - sizeof(struct nlattr))
+				return 0;
+
+			nla = (struct nlattr *)&skb->data[A];
+			if (nla->nla_len > A - skb->len)
+				return 0;
+
+			nla = nla_find_nested(nla, X);
+			if (nla)
+				A = (void *)nla - (void *)skb->data;
+			else
+				A = 0;
+			continue;
+		}
 		default:
 			return 0;
 		}