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

From: Borislav Petkov
Date: Thu Oct 16 2014 - 11:49:15 EST


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@xxxxxxxxxxxxxx>
> ---
> 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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