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Message-ID: <20201221113240.2ae38a77@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date: Mon, 21 Dec 2020 11:32:40 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: weichenchen <weichen.chen@...ux.alibaba.com>
Cc: splendidsky.cwc@...baba-inc.com, yanxu.zw@...baba-inc.com,
"David S. Miller" <davem@...emloft.net>,
Hangbin Liu <liuhangbin@...il.com>,
David Ahern <dsahern@...nel.org>,
Roopa Prabhu <roopa@...ulusnetworks.com>,
Roman Mashak <mrv@...atatu.com>,
Vasily Averin <vvs@...tuozzo.com>,
Jeff Dike <jdike@...mai.com>,
Li RongQing <lirongqing@...du.com>, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: neighbor: fix a crash caused by mod zero
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.chen@...ux.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);
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