Re: [PATCH v2 07/11] arm64: Add skeleton to harden the branch predictor against aliasing attacks

[Will Deacon]

On Fri, Jan 19, 2018 at 11:37:24AM +0800, Li Kun wrote:
> å 2018/1/17 18:07, Will Deacon åé:
> >On Wed, Jan 17, 2018 at 12:10:33PM +0800, Yisheng Xie wrote:
> >>On 2018/1/5 21:12, Will Deacon wrote:
> >>>diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c
> >>>index 5f7097d0cd12..d99b36555a16 100644
> >>>--- a/arch/arm64/mm/context.c
> >>>+++ b/arch/arm64/mm/context.c
> >>>@@ -246,6 +246,8 @@ asmlinkage void post_ttbr_update_workaround(void)
> >>>  			"ic iallu; dsb nsh; isb",
> >>>  			ARM64_WORKAROUND_CAVIUM_27456,
> >>>  			CONFIG_CAVIUM_ERRATUM_27456));
> >>>+
> >>>+	arm64_apply_bp_hardening();
> >>>  }
> >>post_ttbr_update_workaround was used for fix Cavium erratum 2745? so does that
> >>means, if we do not have this erratum, we do not need arm64_apply_bp_hardening()?
> >>when mm_swtich and kernel_exit?
> >>
> >> From the code logical, it seems not only related to erratum 2745 anymore?
> >>should it be renamed?
> >post_ttbr_update_workaround just runs code after a TTBR update, which
> >includes mitigations against variant 2 of "spectre" and also a workaround
> >for a Cavium erratum. These are separate issues.
> But AFAIU, according to the theory of spectre, we don't need to clear the
> BTB every time we return to user?
> If we enable CONFIG_ARM64_SW_TTBR0_PAN, there will be a call to
> arm64_apply_bp_hardening every time kernel exit to el0.
> kernel_exit
>     post_ttbr_update_workaround
>         arm64_apply_bp_hardening

That's a really good point, thanks. What it means is that
post_ttbr_update_workaround is actually the wrong place for this, and we
should be doing it more directly on the switch_mm path -- probably in
check_and_switch_context.

Will