linux-kernel - Re: [RFC 3/5] x86: Add a comment clarifying LDT context switching

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Message-ID: <CALCETrV-2h1H-c7GTzNXvjmT6yzmGzzP6R6hce5QhZbCEi0oKw@mail.gmail.com>
Date:	Mon, 20 Oct 2014 22:44:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Erik Bosman <ebn310@....vu.nl>
Subject: Re: [RFC 3/5] x86: Add a comment clarifying LDT context switching

On Mon, Oct 20, 2014 at 10:41 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Thu, Oct 16, 2014 at 09:21:42AM -0700, Andy Lutomirski wrote:
>> I think it's the same as in the other case in switch_mm. leave_mm does
>> cpumask_clear_cpu(cpu, mm_cpumask(active_mm)), and, once that has
>> happened, modify_ldt won't send an IPI to this CPU. So, if leave_mm
>> runs, and then another CPU calls modify_ldt on the mm that is in lazy
>> mode here, it won't update our LDT register, so the LDT register and
>> prev->context.ldt might not match.
>
> Ok, let me see if I can follow with an example:
>
> We call leave_mm() on, say, cpu 3 and mm_cpumask(active_mm) has cpu 3 and
> 4 set. Then, on cpu 4 we call modify_ldt on that same mm and there in
> alloc_ldt() we have this:
>
>                 if (!cpumask_equal(mm_cpumask(current->mm),
>                                    cpumask_of(smp_processor_id())))
>                         smp_call_function(flush_ldt, current->mm, 1);
>
> and since we've cleared cpu 3 from the cpumask, we don't flush_ldt()
> on it and there you have the difference.
>
> Am I close?

You're exactly correct, or at least you seem to understand it the way I do :)

--Andy
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