packet: avoid panic in packet_getsockopt()

syzkaller got crashes in packet_getsockopt() processing
PACKET_ROLLOVER_STATS command while another thread was managing
to change po->rollover

Using RCU will fix this bug. We might later add proper RCU annotations
for sparse sake.

In v2: I replaced kfree(rollover) in fanout_add() to kfree_rcu()
variant, as spotted by John.

Fixes: a9b6391814d5 ("packet: rollover statistics")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: John Sperbeck <jsperbeck@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1769,7 +1769,7 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 
 out:
 	if (err && rollover) {
-		kfree(rollover);
+		kfree_rcu(rollover, rcu);
 		po->rollover = NULL;
 	}
 	mutex_unlock(&fanout_mutex);
@@ -1796,8 +1796,10 @@ static struct packet_fanout *fanout_release(struct sock *sk)
 		else
 			f = NULL;
 
-		if (po->rollover)
+		if (po->rollover) {
 			kfree_rcu(po->rollover, rcu);
+			po->rollover = NULL;
+		}
 	}
 	mutex_unlock(&fanout_mutex);
 
@@ -3851,6 +3853,7 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 	void *data = &val;
 	union tpacket_stats_u st;
 	struct tpacket_rollover_stats rstats;
+	struct packet_rollover *rollover;
 
 	if (level != SOL_PACKET)
 		return -ENOPROTOOPT;
@@ -3929,13 +3932,18 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 		       0);
 		break;
 	case PACKET_ROLLOVER_STATS:
-		if (!po->rollover)
+		rcu_read_lock();
+		rollover = rcu_dereference(po->rollover);
+		if (rollover) {
+			rstats.tp_all = atomic_long_read(&rollover->num);
+			rstats.tp_huge = atomic_long_read(&rollover->num_huge);
+			rstats.tp_failed = atomic_long_read(&rollover->num_failed);
+			data = &rstats;
+			lv = sizeof(rstats);
+		}
+		rcu_read_unlock();
+		if (!rollover)
 			return -EINVAL;
-		rstats.tp_all = atomic_long_read(&po->rollover->num);
-		rstats.tp_huge = atomic_long_read(&po->rollover->num_huge);
-		rstats.tp_failed = atomic_long_read(&po->rollover->num_failed);
-		data = &rstats;
-		lv = sizeof(rstats);
 		break;
 	case PACKET_TX_HAS_OFF:
 		val = po->tp_tx_has_off;