Messages in this thread | | | Date | Mon, 8 Jan 2018 17:06:24 +0000 | From | Will Deacon <> | Subject | Re: [v2,03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3 |
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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@arm.com> > > Signed-off-by: Will Deacon <will.deacon@arm.com> > > --- > > 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
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