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

[Li Kun]

Hi will,


å 2018/1/17 18:07, Will Deacon åé:
> On Wed, Jan 17, 2018 at 12:10:33PM +0800, Yisheng Xie wrote:
> > Hi Will,
> >
> > 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
> Will
>
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--
Best Regards
Li Kun