linux-kernel - Re: [PATCH v2 8/8] x86/mm: Only flush indirect branches when switching into non dumpable process

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Date:   Sun, 21 Jan 2018 11:33:13 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, bp@...en8.de, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: Re: [PATCH v2 8/8] x86/mm: Only flush indirect branches when
 switching into non dumpable process


* David Woodhouse <dwmw@...zon.co.uk> wrote:

> From: Andi Kleen <ak@...ux.intel.com>
> 
> Flush indirect branches when switching into a process that marked
> itself non dumpable.  This protects high value processes like gpg
> better, without having too high performance overhead.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> ---
>  arch/x86/mm/tlb.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index 304de7d..f64e80c 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -225,8 +225,19 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>  		 * Avoid user/user BTB poisoning by flushing the branch predictor
>  		 * when switching between processes. This stops one process from
>  		 * doing Spectre-v2 attacks on another.
> +		 *
> +		 * As an optimization: Flush indirect branches only when
> +		 * switching into processes that disable dumping.
> +		 *
> +		 * This will not flush when switching into kernel threads.
> +		 * But it would flush when switching into idle and back
> +		 *
> +		 * It might be useful to have a one-off cache here
> +		 * to also not flush the idle case, but we would need some
> +		 * kind of stable sequence number to remember the previous mm.

Punctuation and grammar is pretty inconsistent, please change it to something more 
readable, like:

		 * As an optimization flush indirect branches only when
		 * switching into processes that disable dumping.
		 *
		 * This will not flush branches when switching into kernel 
		 * threads, but it would flush them when switching to the
		 * idle thread and back.
		 *
		 * It might be useful to have a one-off cache here
		 * to also not flush the idle case, but we would need some
		 * kind of stable sequence number to remember the previous mm.


Thanks,

	Ingo