packet: avoid out of bounds read in round robin fanout

[ Upstream commit 468479e6043c84f5a65299cc07cb08a22a28c2b1 ]

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. Ensure
that the return value is always < num.

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>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 16 deletions

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 131545a06f05..fe1610ddeacf 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 +1283,9 @@ static unsigned int fanout_demux_lb(struct packet_fanout *f,
                                     struct sk_buff *skb,
                                     unsigned int num)
 {
-        int cur, old;
+        unsigned int val = atomic_inc_return(&f->rr_cur);
 
-        cur = atomic_read(&f->rr_cur);
-        while ((old = atomic_cmpxchg(&f->rr_cur, cur,
-                                     fanout_rr_next(f, num))) != cur)
-                cur = old;
-        return cur;
+        return val % num;
 }
 
 static unsigned int fanout_demux_cpu(struct packet_fanout *f,