netfilter: use get_random_u32 instead of prandom

bh might occur while updating per-cpu rnd_state from user context,
ie. local_out path.

BUG: using smp_processor_id() in preemptible [00000000] code: nginx/2725
caller is nft_ng_random_eval+0x24/0x54 [nft_numgen]
Call Trace:
check_preemption_disabled+0xde/0xe0
nft_ng_random_eval+0x24/0x54 [nft_numgen]

Use the random driver instead, this also avoids need for local prandom
state. Moreover, prandom now uses the random driver since d4150779e60f
("random32: use real rng for non-deterministic randomness").

Based on earlier patch from Pablo Neira.

Fixes: 6b2faee0ca91 ("netfilter: nft_meta: place prandom handling in a helper")
Fixes: 978d8f9055c3 ("netfilter: nft_numgen: add map lookups for numgen random operations")
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_meta.c

···
 #include <linux/in.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
+#include <linux/random.h>
 #include <linux/smp.h>
 #include <linux/static_key.h>
 #include <net/dst.h>
···
 #define NFT_META_SECS_PER_HOUR		3600
 #define NFT_META_SECS_PER_DAY		86400
 #define NFT_META_DAYS_PER_WEEK		7
-
-static DEFINE_PER_CPU(struct rnd_state, nft_prandom_state);
 
 static u8 nft_meta_weekday(void)
 {
···
 	return true;
 }
 
-static noinline u32 nft_prandom_u32(void)
-{
-	struct rnd_state *state = this_cpu_ptr(&nft_prandom_state);
-
-	return prandom_u32_state(state);
-}
-
 #ifdef CONFIG_IP_ROUTE_CLASSID
 static noinline bool
 nft_meta_get_eval_rtclassid(const struct sk_buff *skb, u32 *dest)
···
 		break;
 #endif
 	case NFT_META_PRANDOM:
-		*dest = nft_prandom_u32();
+		*dest = get_random_u32();
 		break;
 #ifdef CONFIG_XFRM
 	case NFT_META_SECPATH:
···
 		len = IFNAMSIZ;
 		break;
 	case NFT_META_PRANDOM:
-		prandom_init_once(&nft_prandom_state);
 		len = sizeof(u32);
 		break;
 #ifdef CONFIG_XFRM

net/netfilter/nft_numgen.c

···
 #include <linux/netlink.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
+#include <linux/random.h>
 #include <linux/static_key.h>
 #include <net/netfilter/nf_tables.h>
 #include <net/netfilter/nf_tables_core.h>
-
-static DEFINE_PER_CPU(struct rnd_state, nft_numgen_prandom_state);
 
 struct nft_ng_inc {
 	u8			dreg;
···
 	u32			offset;
 };
 
-static u32 nft_ng_random_gen(struct nft_ng_random *priv)
+static u32 nft_ng_random_gen(const struct nft_ng_random *priv)
 {
-	struct rnd_state *state = this_cpu_ptr(&nft_numgen_prandom_state);
-
-	return reciprocal_scale(prandom_u32_state(state), priv->modulus) +
-	       priv->offset;
+	return reciprocal_scale(get_random_u32(), priv->modulus) + priv->offset;
 }
 
 static void nft_ng_random_eval(const struct nft_expr *expr,
···
 
 	if (priv->offset + priv->modulus - 1 < priv->offset)
 		return -EOVERFLOW;
-
-	prandom_init_once(&nft_numgen_prandom_state);
 
 	return nft_parse_register_store(ctx, tb[NFTA_NG_DREG], &priv->dreg,
 					NULL, NFT_DATA_VALUE, sizeof(u32));