On Tue, Jan 23, 2018 at 12:28:03PM +0000, Suzuki K Poulose wrote:
> KPTI capability is a security feature which should be enabled
> when at least one CPU on the system needs it. Any late CPU
> which needs the kernel support, should be prevented from
> booting (and thus making the system unsecure) if the feature
> was not already enabled.
Is there an actual change to behaviour here?
It's not very obvious from the commit message, or the patch when read in
isolation.
>
> Cc: Will Deacon <will.dea...@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poul...@arm.com>
> ---
> arch/arm64/kernel/cpufeature.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 111f6c4b4cd7..2627a836e99d 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -862,9 +862,8 @@ static bool has_no_fpsimd(const struct
> arm64_cpu_capabilities *entry, int __unus
> 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,
[Nit: All the other .matches functions have a predicate-style name, so
"should_unmap_kernel_at_el0" or "must_unmap_kernel_at_el0" might be
better names... However, the name wasn't introduced by this series and
it's not a huge deal.]
> - int __unused)
> + int scope)
> {
> - u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
>
> /* Forced on command line? */
> if (__kpti_forced) {
> @@ -878,8 +877,7 @@ static bool unmap_kernel_at_el0(const struct
> arm64_cpu_capabilities *entry,
> return true;
>
> /* Defer to CPU feature registers */
> - return !cpuid_feature_extract_unsigned_field(pfr0,
> - ID_AA64PFR0_CSV3_SHIFT);
> + return !has_cpuid_feature(entry, scope);
> }
>
> static int __init parse_kpti(char *str)
> @@ -1003,7 +1001,10 @@ static const struct arm64_cpu_capabilities
> arm64_features[] = {
> {
> .desc = "Kernel page table isolation (KPTI)",
> .capability = ARM64_UNMAP_KERNEL_AT_EL0,
> - .type = ARM64_CPUCAP_BOOT_SYSTEM_FEATURE,
> + .type = ARM64_CPUCAP_STRICT_CPU_LOCAL_FEATURE,
> + .sys_reg = SYS_ID_AA64PFR0_EL1,
> + .field_pos = ID_AA64PFR0_CSV3_SHIFT,
> + .min_field_value = 1,
> .matches = unmap_kernel_at_el0,
> },
> #endif
[...]
Cheers
---Dave
