On Sat, 2018-01-20 at 20:22 +0100, KarimAllah Ahmed wrote:
> From: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
I think this is probably From: Andi now rather than From: Tim?
We do need the series this far in order to have a full retpoline-based
mitigation, and I'd like to see that go in sooner rather than later.
There's a little more discussion to be had about the IBRS parts which
come later in the series (and the final one or two which weren't posted
yet).
I think this is the one patch of the "we want this now" IBPB set that
we expect serious debate on, which is why it's still a separate
"optimisation" patch on top of the previous one which just does IBPB
unconditionally.
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> ---
> 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.
> */
> - indirect_branch_prediction_barrier();
> + if (tsk && tsk->mm && get_dumpable(tsk->mm) != SUID_DUMP_USER)
> + indirect_branch_prediction_barrier();
>
> if (IS_ENABLED(CONFIG_VMAP_STACK)) {
> /*Attachment:
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