On Mon, 21 Dec 2020 21:07:44 +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.c...@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.
In terms of not breaking abstraction? The decision to call
this helper or not is made in the caller. And both callers
do a NEIGH_VAR(p, PROXY_DELAY) == 0 check before making the
call.
It seems like if the caller used READ_ONCE and passed the value
in we would save ourselves the potentially surprising code flow.
> 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);
