[PATCH net] packet: avoid out of bounds read in round robin fanout

[Willem de Bruijn <willemb@google.com>]

From: Willem de Bruijn <willemb@google.com>

PACKET_FANOUT_LB computes f->rr_cur such that it is modulo
f->num_members. It returns the old value unconditionally, but
f->num_members may have changed since the last store. This can be
fixed with

  -        return cur
  +        return cur < num ? : 0;

When modifying the logic, simplify it further by replacing the loop
with an unconditional atomic increment.

Fixes: dc99f600698d ("packet: Add fanout support.")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
 net/packet/af_packet.c | 19 ++-----------------
 1 file changed, 2 insertions(+), 17 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index b5989c6..efd35e8 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1272,16 +1272,6 @@ static void packet_sock_destruct(struct sock *sk)
 	sk_refcnt_debug_dec(sk);
 }
 
-static int fanout_rr_next(struct packet_fanout *f, unsigned int num)
-{
-	int x = atomic_read(&f->rr_cur) + 1;
-
-	if (x >= num)
-		x = 0;
-
-	return x;
-}
-
 static unsigned int fanout_demux_hash(struct packet_fanout *f,
 				      struct sk_buff *skb,
 				      unsigned int num)
@@ -1293,13 +1283,8 @@ static unsigned int fanout_demux_lb(struct packet_fanout *f,
 				    struct sk_buff *skb,
 				    unsigned int num)
 {
-	int cur, old;
-
-	cur = atomic_read(&f->rr_cur);
-	while ((old = atomic_cmpxchg(&f->rr_cur, cur,
-				     fanout_rr_next(f, num))) != cur)
-		cur = old;
-	return cur;
+	unsigned int val = atomic_inc_return(&f->rr_cur);
+	return val % num;
 }
 
 static unsigned int fanout_demux_cpu(struct packet_fanout *f,
-- 
2.2.0.rc0.207.ga3a616c