[PATCH net] packet: avoid panic in packet_getsockopt()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

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.

Fixes: a9b6391814d5 ("packet: rollover statistics")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Willem de Bruijn <will...@google.com>
---
 net/packet/af_packet.c |   22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 
bec01a3daf5b02bd716dbff5c9efef8d6a7982be..1d8a7add86b4f29880e11c6f4971d79319dcb426
 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -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;