From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] ipv6: fix signedness of tmp_prefered_lft underflow
 check
Date: Wed, 19 Oct 2016 14:58:16 -0400 (EDT)
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From: Jiri Bohac <jbohac@suse.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.