Re: [v2,03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3

[Will Deacon]

On Sun, Jan 07, 2018 at 11:24:02PM -0800, Jayachandran C wrote:
> On Fri, Jan 05, 2018 at 01:12:33PM +0000, Will Deacon wrote:
> > For non-KASLR kernels where the KPTI behaviour has not been overridden
> > on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
> > or not we should unmap the kernel whilst running at EL0.
> > 
> > Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> > ---
> >  arch/arm64/include/asm/sysreg.h | 1 +
> >  arch/arm64/kernel/cpufeature.c  | 8 +++++++-
> >  2 files changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> > index 08cc88574659..ae519bbd3f9e 100644
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -437,6 +437,7 @@
> >  #define ID_AA64ISAR1_DPB_SHIFT		0
> >  
> >  /* id_aa64pfr0 */
> > +#define ID_AA64PFR0_CSV3_SHIFT		60
> >  #define ID_AA64PFR0_SVE_SHIFT		32
> >  #define ID_AA64PFR0_GIC_SHIFT		24
> >  #define ID_AA64PFR0_ASIMD_SHIFT		20
> > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > index 9f0545dfe497..d723fc071f39 100644
> > --- a/arch/arm64/kernel/cpufeature.c
> > +++ b/arch/arm64/kernel/cpufeature.c
> > @@ -145,6 +145,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
> >  };
> >  
> >  static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
> > +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
> >  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
> >  	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
> >  	S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
> > @@ -851,6 +852,8 @@ static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
> >  static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> >  				int __unused)
> >  {
> > +	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> > +
> >  	/* Forced on command line? */
> >  	if (__kpti_forced) {
> >  		pr_info_once("kernel page table isolation forced %s by command line option\n",
> > @@ -862,7 +865,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> >  	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> >  		return true;
> >  
> > -	return false;
> > +	/* Defer to CPU feature registers */
> > +	return !cpuid_feature_extract_unsigned_field(pfr0,
> > +						     ID_AA64PFR0_CSV3_SHIFT);
> 
> If I read this correctly, this enables KPTI on all processors without the CSV3
> set (which seems to be a future capability).
> 
> Turning on KPTI has a small but significant overhead, so I think we should turn
> it off on processors that are not vulnerable to CVE-2017-5754. Can we add something
> like  this:
> 
> --->8
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 19ed09b..202b037 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -862,6 +862,13 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>                 return __kpti_forced > 0;
>         }
>  
> +       /* Don't force KPTI for CPUs that are not vulnerable */
> +       switch (read_cpuid_id() & MIDR_CPU_MODEL_MASK) {
> +               case MIDR_CAVIUM_THUNDERX2:
> +               case MIDR_BRCM_VULCAN:
> +                       return false;
> +       }
> +

KASLR aside (I agree with Marc on that), I did consider an MIDR whitelist,
but it gets nasty for big.LITTLE systems if maxcpus= is used and we see a
non-whitelisted CPU after we've booted. At this point, we can't actually
bring the thing online.

You could make the argument that if you're passing maxcpus= then you can just
easily pass kpti= as well, but I wasn't sure.

Will