Messages in this thread | | | Date | Sat, 19 Dec 2020 10:21:16 -0800 | From | Jakub Kicinski <> | Subject | Re: [PATCH] net: neighbor: fix a crash caused by mod zero |
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On Fri, 18 Dec 2020 12:20:19 +0800 weichenchen wrote: > 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>
Let's have the caller pass in the value since it did the checking?
> diff --git a/net/core/neighbour.c b/net/core/neighbour.c > index 9500d28a43b0..eb5d015c53d3 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 = NEIGH_VAR(p, PROXY_DELAY); > + > + if (delay <= 0)
Not that this still doesn't guarantee that the compiler won't re-read the value (however unlikely). We need a READ_ONCE().
> + sched_next = now; > + else > + sched_next = now + (prandom_u32() % delay); > > if (tbl->proxy_queue.qlen > NEIGH_VAR(p, PROXY_QLEN)) { > kfree_skb(skb);
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