[PATCH v2] 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 double-checks NEIGH_VAR(p, PROXY_DELAY) in
pneigh_enqueue() to ensure not to take zero as modulus.

Signed-off-by: weichenchen <weichen.chen@linux.alibaba.com>
---
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.

Note: I don't think having the caller pass in the value is a
good idea mainly because delay should be only decided by
/proc/sys/net/ipv4/neigh/[device]/proxy_delay rather than the
caller.
---
 net/core/neighbour.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 9500d28a43b0..7b03d3f129c0 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1570,9 +1570,14 @@ void pneigh_enqueue(struct neigh_table *tbl, struct neigh_parms *p,
 		    struct sk_buff *skb)
 {
 	unsigned long now = jiffies;
+	unsigned long sched_next;
 
-	unsigned long sched_next = now + (prandom_u32() %
-					  NEIGH_VAR(p, PROXY_DELAY));
+	int delay = READ_ONCE(NEIGH_VAR(p, PROXY_DELAY));
+
+	if (unlikely(delay <= 0))
+		sched_next = now;
+	else
+		sched_next = now + (prandom_u32() % delay);
 
 	if (tbl->proxy_queue.qlen > NEIGH_VAR(p, PROXY_QLEN)) {
 		kfree_skb(skb);
-- 
2.20.1 (Apple Git-117)