neighbour: fix various data-races

1) tbl->gc_thresh1, tbl->gc_thresh2, tbl->gc_thresh3 and tbl->gc_interval
   can be written from sysfs.

2) tbl->last_flush is read locklessly from neigh_alloc()

3) tbl->proxy_queue.qlen is read locklessly from neightbl_fill_info()

4) neightbl_fill_info() reads cpu stats that can be changed concurrently.

Fixes: c7fb64db ("[NETLINK]: Neighbour table configuration and statistics via rtnetlink")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231019122104.1448310-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/neighbour.c

@@ -251,7 +251,8 @@ bool neigh_remove_one(struct neighbour *ndel, struct neigh_table *tbl)
 
 static int neigh_forced_gc(struct neigh_table *tbl)
 {
-	int max_clean = atomic_read(&tbl->gc_entries) - tbl->gc_thresh2;
+	int max_clean = atomic_read(&tbl->gc_entries) -
+			READ_ONCE(tbl->gc_thresh2);
 	unsigned long tref = jiffies - 5 * HZ;
 	struct neighbour *n, *tmp;
 	int shrunk = 0;
@@ -280,7 +281,7 @@ static int neigh_forced_gc(struct neigh_table *tbl)
 		}
 	}
 
-	tbl->last_flush = jiffies;
+	WRITE_ONCE(tbl->last_flush, jiffies);
 
 	write_unlock_bh(&tbl->lock);
 
@@ -464,17 +465,17 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl,
 {
 	struct neighbour *n = NULL;
 	unsigned long now = jiffies;
-	int entries;
+	int entries, gc_thresh3;
 
 	if (exempt_from_gc)
 		goto do_alloc;
 
 	entries = atomic_inc_return(&tbl->gc_entries) - 1;
-	if (entries >= tbl->gc_thresh3 ||
-	    (entries >= tbl->gc_thresh2 &&
-	     time_after(now, tbl->last_flush + 5 * HZ))) {
-		if (!neigh_forced_gc(tbl) &&
-		    entries >= tbl->gc_thresh3) {
+	gc_thresh3 = READ_ONCE(tbl->gc_thresh3);
+	if (entries >= gc_thresh3 ||
+	    (entries >= READ_ONCE(tbl->gc_thresh2) &&
+	     time_after(now, READ_ONCE(tbl->last_flush) + 5 * HZ))) {
+		if (!neigh_forced_gc(tbl) && entries >= gc_thresh3) {
 			net_info_ratelimited("%s: neighbor table overflow!\n",
 					     tbl->id);
 			NEIGH_CACHE_STAT_INC(tbl, table_fulls);
@@ -955,13 +956,14 @@ static void neigh_periodic_work(struct work_struct *work)
 
 	if (time_after(jiffies, tbl->last_rand + 300 * HZ)) {
 		struct neigh_parms *p;
-		tbl->last_rand = jiffies;
+
+		WRITE_ONCE(tbl->last_rand, jiffies);
 		list_for_each_entry(p, &tbl->parms_list, list)
 			p->reachable_time =
 				neigh_rand_reach_time(NEIGH_VAR(p, BASE_REACHABLE_TIME));
 	}
 
-	if (atomic_read(&tbl->entries) < tbl->gc_thresh1)
+	if (atomic_read(&tbl->entries) < READ_ONCE(tbl->gc_thresh1))
 		goto out;
 
 	for (i = 0 ; i < (1 << nht->hash_shift); i++) {
@@ -2167,15 +2169,16 @@ static int neightbl_fill_info(struct sk_buff *skb, struct neigh_table *tbl,
 	ndtmsg->ndtm_pad2   = 0;
 
 	if (nla_put_string(skb, NDTA_NAME, tbl->id) ||
-	    nla_put_msecs(skb, NDTA_GC_INTERVAL, tbl->gc_interval, NDTA_PAD) ||
-	    nla_put_u32(skb, NDTA_THRESH1, tbl->gc_thresh1) ||
-	    nla_put_u32(skb, NDTA_THRESH2, tbl->gc_thresh2) ||
-	    nla_put_u32(skb, NDTA_THRESH3, tbl->gc_thresh3))
+	    nla_put_msecs(skb, NDTA_GC_INTERVAL, READ_ONCE(tbl->gc_interval),
+			  NDTA_PAD) ||
+	    nla_put_u32(skb, NDTA_THRESH1, READ_ONCE(tbl->gc_thresh1)) ||
+	    nla_put_u32(skb, NDTA_THRESH2, READ_ONCE(tbl->gc_thresh2)) ||
+	    nla_put_u32(skb, NDTA_THRESH3, READ_ONCE(tbl->gc_thresh3)))
 		goto nla_put_failure;
 	{
 		unsigned long now = jiffies;
-		long flush_delta = now - tbl->last_flush;
-		long rand_delta = now - tbl->last_rand;
+		long flush_delta = now - READ_ONCE(tbl->last_flush);
+		long rand_delta = now - READ_ONCE(tbl->last_rand);
 		struct neigh_hash_table *nht;
 		struct ndt_config ndc = {
 			.ndtc_key_len		= tbl->key_len,
@@ -2183,7 +2186,7 @@ static int neightbl_fill_info(struct sk_buff *skb, struct neigh_table *tbl,
 			.ndtc_entries		= atomic_read(&tbl->entries),
 			.ndtc_last_flush	= jiffies_to_msecs(flush_delta),
 			.ndtc_last_rand		= jiffies_to_msecs(rand_delta),
-			.ndtc_proxy_qlen	= tbl->proxy_queue.qlen,
+			.ndtc_proxy_qlen	= READ_ONCE(tbl->proxy_queue.qlen),
 		};
 
 		rcu_read_lock();
@@ -2206,17 +2209,17 @@ static int neightbl_fill_info(struct sk_buff *skb, struct neigh_table *tbl,
 			struct neigh_statistics	*st;
 
 			st = per_cpu_ptr(tbl->stats, cpu);
-			ndst.ndts_allocs		+= st->allocs;
-			ndst.ndts_destroys		+= st->destroys;
-			ndst.ndts_hash_grows		+= st->hash_grows;
-			ndst.ndts_res_failed		+= st->res_failed;
-			ndst.ndts_lookups		+= st->lookups;
-			ndst.ndts_hits			+= st->hits;
-			ndst.ndts_rcv_probes_mcast	+= st->rcv_probes_mcast;
-			ndst.ndts_rcv_probes_ucast	+= st->rcv_probes_ucast;
-			ndst.ndts_periodic_gc_runs	+= st->periodic_gc_runs;
-			ndst.ndts_forced_gc_runs	+= st->forced_gc_runs;
-			ndst.ndts_table_fulls		+= st->table_fulls;
+			ndst.ndts_allocs		+= READ_ONCE(st->allocs);
+			ndst.ndts_destroys		+= READ_ONCE(st->destroys);
+			ndst.ndts_hash_grows		+= READ_ONCE(st->hash_grows);
+			ndst.ndts_res_failed		+= READ_ONCE(st->res_failed);
+			ndst.ndts_lookups		+= READ_ONCE(st->lookups);
+			ndst.ndts_hits			+= READ_ONCE(st->hits);
+			ndst.ndts_rcv_probes_mcast	+= READ_ONCE(st->rcv_probes_mcast);
+			ndst.ndts_rcv_probes_ucast	+= READ_ONCE(st->rcv_probes_ucast);
+			ndst.ndts_periodic_gc_runs	+= READ_ONCE(st->periodic_gc_runs);
+			ndst.ndts_forced_gc_runs	+= READ_ONCE(st->forced_gc_runs);
+			ndst.ndts_table_fulls		+= READ_ONCE(st->table_fulls);
 		}
 
 		if (nla_put_64bit(skb, NDTA_STATS, sizeof(ndst), &ndst,
@@ -2445,16 +2448,16 @@ static int neightbl_set(struct sk_buff *skb, struct nlmsghdr *nlh,
 		goto errout_tbl_lock;
 
 	if (tb[NDTA_THRESH1])
-		tbl->gc_thresh1 = nla_get_u32(tb[NDTA_THRESH1]);
+		WRITE_ONCE(tbl->gc_thresh1, nla_get_u32(tb[NDTA_THRESH1]));
 
 	if (tb[NDTA_THRESH2])
-		tbl->gc_thresh2 = nla_get_u32(tb[NDTA_THRESH2]);
+		WRITE_ONCE(tbl->gc_thresh2, nla_get_u32(tb[NDTA_THRESH2]));
 
 	if (tb[NDTA_THRESH3])
-		tbl->gc_thresh3 = nla_get_u32(tb[NDTA_THRESH3]);
+		WRITE_ONCE(tbl->gc_thresh3, nla_get_u32(tb[NDTA_THRESH3]));
 
 	if (tb[NDTA_GC_INTERVAL])
-		tbl->gc_interval = nla_get_msecs(tb[NDTA_GC_INTERVAL]);
+		WRITE_ONCE(tbl->gc_interval, nla_get_msecs(tb[NDTA_GC_INTERVAL]));
 
 	err = 0;