netdev - Re: [PATCH] ipv6: fix signedness of tmp_prefered

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Message-Id: <20161019.145816.259194714319437266.davem@davemloft.net>
Date:   Wed, 19 Oct 2016 14:58:16 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     jbohac@...e.cz
Cc:     julia.lawall@...6.fr, kuznet@....inr.ac.ru, jmorris@...ei.org,
        yoshfuji@...ux-ipv6.org, kaber@...sh.net, netdev@...r.kernel.org,
        kbuild-all@...org
Subject: Re: [PATCH] ipv6: fix signedness of tmp_prefered_lft underflow
 check

From: Jiri Bohac <jbohac@...e.cz>
Date: Wed, 19 Oct 2016 15:16:36 +0200

> The purpose was to guard against the user updating the
> temp_prefered_lft sysctl after this:
> 
>         max_desync_factor = min_t(__u32,
>                                   idev->cnf.max_desync_factor,
>                                   idev->cnf.temp_prefered_lft - regen_advance);
> 
> but before this:
> 
> 	tmp_prefered_lft = idev->cnf.temp_prefered_lft + age -
> 			    idev->desync_factor;

That's a different problem.

Read the sysctl values of interest into local variables using
READ_ONCE() before the calculations, that way the situation your
describe is impossible.