linux-kernel - Re: [PATCH v4] net: neighbor: fix a crash caused by mod zero

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Message-Id: <20201228.145128.1498314185351532341.davem@davemloft.net>
Date:   Mon, 28 Dec 2020 14:51:28 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     weichen.chen@...ux.alibaba.com
Cc:     eric.dumazet@...il.com, kuba@...nel.org,
        splendidsky.cwc@...baba-inc.com, yanxu.zw@...baba-inc.com,
        dsahern@...nel.org, liuhangbin@...il.com,
        roopa@...ulusnetworks.com, jdike@...mai.com,
        nikolay@...ulusnetworks.com, lirongqing@...du.com,
        mrv@...atatu.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] net: neighbor: fix a crash caused by mod zero

From: weichenchen <weichen.chen@...ux.alibaba.com>
Date: Fri, 25 Dec 2020 13:44:45 +0800

> 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@...ux.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.

Applied and queued up for -stable, thanks.