[PATCH v4] net: neighbor: fix a crash caused by mod zero

[weichenchen <weichen.chen@linux.alibaba.com>]

pneigh_enqueue() tries to obtain a random delay by mod
NEIGH_VAR(p, PROXY_DELAY). However, NEIGH_VAR(p, PROXY_DELAY)
migth be zero at that point because someone could write zero
to /proc/sys/net/ipv4/neigh/[device]/proxy_delay after the
callers check it.

This patch uses prandom_u32_max() to get a random delay instead
which avoids potential division by zero.

Signed-off-by: weichenchen <weichen.chen@linux.alibaba.com>
---
V4:
    - Use prandom_u32_max() to get a random delay in
      pneigh_enqueue().
V3:
    - Callers need to pass the delay time to pneigh_enqueue()
      now and they should guarantee it is not zero.
    - Use READ_ONCE() to read NEIGH_VAR(p, PROXY_DELAY) in both
      of the existing callers of pneigh_enqueue() and then pass
      it to pneigh_enqueue().
V2:
    - Use READ_ONCE() to prevent the complier from re-reading
      NEIGH_VAR(p, PROXY_DELAY).
    - Give a hint to the complier that delay <= 0 is unlikely
      to happen.

V4 is quite concise and works well.
Thanks for Eric's and Jakub's advice.
---
 net/core/neighbour.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 9500d28a43b0..277ed854aef1 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1569,10 +1569,8 @@ static void neigh_proxy_process(struct timer_list *t)
 void pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 		    struct sk_buff *skb)
 {
-	unsigned long now = jiffies;
-
-	unsigned long sched_next = now + (prandom_u32() %
-					  NEIGH_VAR(p, PROXY_DELAY));
+	unsigned long sched_next = jiffies +
+			prandom_u32_max(NEIGH_VAR(p, PROXY_DELAY));
 
 	if (tbl->proxy_queue.qlen > NEIGH_VAR(p, PROXY_QLEN)) {
 		kfree_skb(skb);
-- 
2.20.1 (Apple Git-117)