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SubjectRe: [PATCH v2 8/8] x86/mm: Only flush indirect branches when switching into non dumpable process

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

> From: Andi Kleen <ak@linux.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@linux.intel.com>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: KarimAllah Ahmed <karahmed@amazon.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

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