netfilter: meta: add PRANDOM support

Can be used to randomly match packets e.g. for statistic traffic sampling.

See commit 3ad0040573b0c00f8848
("bpf: split state from prandom_u32() and consolidate {c, e}BPF prngs")
for more info why this doesn't use prandom_u32 directly.

Unlike bpf nft_meta can be built as a module, so add an EXPORT_SYMBOL
for prandom_seed_full_state too.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/netfilter/nf_tables.h

@@ -681,6 +681,7 @@ enum nft_exthdr_attributes {
  * @NFT_META_IIFGROUP: packet input interface group
  * @NFT_META_OIFGROUP: packet output interface group
  * @NFT_META_CGROUP: socket control group (skb->sk->sk_classid)
+ * @NFT_META_PRANDOM: a 32bit pseudo-random number
  */
 enum nft_meta_keys {
 	NFT_META_LEN,
@@ -707,6 +708,7 @@ enum nft_meta_keys {
 	NFT_META_IIFGROUP,
 	NFT_META_OIFGROUP,
 	NFT_META_CGROUP,
+	NFT_META_PRANDOM,
 };
 
 /**

lib/random32.c

@@ -255,6 +255,7 @@ void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state)
 		prandom_warmup(state);
 	}
 }
+EXPORT_SYMBOL(prandom_seed_full_state);
 
 /*
  *	Generate better values after random number generator

net/netfilter/nft_meta.c

@@ -28,6 +28,8 @@
 
 #include <uapi/linux/netfilter_bridge.h> /* NF_BR_PRE_ROUTING */
 
+static DEFINE_PER_CPU(struct rnd_state, nft_prandom_state);
+
 void nft_meta_get_eval(const struct nft_expr *expr,
 		       struct nft_regs *regs,
 		       const struct nft_pktinfo *pkt)
@@ -181,6 +183,11 @@ void nft_meta_get_eval(const struct nft_expr *expr,
 		*dest = sock_cgroup_classid(&sk->sk_cgrp_data);
 		break;
 #endif
+	case NFT_META_PRANDOM: {
+		struct rnd_state *state = this_cpu_ptr(&nft_prandom_state);
+		*dest = prandom_u32_state(state);
+		break;
+	}
 	default:
 		WARN_ON(1);
 		goto err;
@@ -277,6 +284,10 @@ int nft_meta_get_init(const struct nft_ctx *ctx,
 	case NFT_META_OIFNAME:
 		len = IFNAMSIZ;
 		break;
+	case NFT_META_PRANDOM:
+		prandom_init_once(&nft_prandom_state);
+		len = sizeof(u32);
+		break;
 	default:
 		return -EOPNOTSUPP;
 	}