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.poul...@arm.com> > Signed-off-by: Will Deacon <will.dea...@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; + } + /* Useful for KASLR robustness */ if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) return true; -- JC