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

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Message-ID: <20141016154914.GD30314@nazgul.tnic>
Date:	Thu, 16 Oct 2014 17:49:14 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
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 Tue, Oct 14, 2014 at 03:57:37PM -0700, Andy Lutomirski wrote:
> The code is correct, but only for a rather subtle reason.  This
> confused me for quite a while when I read switch_mm, so clarify the
> code to avoid confusing other people, too.
> 
> TBH, I wouldn't be surprised if this code was only correct by
> accident.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/include/asm/mmu_context.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> index 166af2a8e865..04478103df37 100644
> --- a/arch/x86/include/asm/mmu_context.h
> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -53,7 +53,16 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>  		/* Stop flush ipis for the previous mm */
>  		cpumask_clear_cpu(cpu, mm_cpumask(prev));
>  
> -		/* Load the LDT, if the LDT is different: */
> +		/*
> +		 * Load the LDT, if the LDT is different.
> +		 *
> +		 * It's possible leave_mm(prev) has been called.  If so,
> +		 * then prev->context.ldt could be out of sync with the
> +		 * LDT descriptor or the LDT register.  This can only happen

I'm staring at the code and trying to figure out where on the leave_mm()
path this could happen. Got any code pointers?

:-)

> +		 * if prev->context.ldt is non-null, since we never free
> +		 * an LDT.  But LDTs can't be shared across mms, so
> +		 * prev->context.ldt won't be equal to next->context.ldt.
> +		 */
>  		if (unlikely(prev->context.ldt != next->context.ldt))
>  			load_LDT_nolock(&next->context);
>  	}
> -- 
> 1.9.3
> 
> --
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-- 
Regards/Gruss,
    Boris.
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