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    SubjectRe: [PATCH v2 10/20] arm64: capabilities: Restrict KPTI detection to boot-time CPUs
    On Wed, Jan 31, 2018 at 06:27:57PM +0000, Suzuki K Poulose wrote:
    > KPTI is treated as a system wide feature, where we enable the feature
    > when all the CPUs on the system suffers from the security vulnerability,

    Should that be "when any CPU"?

    > unless it is forced via kernel command line. Also, if a late CPU needs
    > KPTI but KPTI was not enabled at boot time, the CPU is currently allowed
    > to boot, which is a potential security vulnerability. This patch ensures
    > that late CPUs are rejected as appropriate if they need KPTI but it wasn't
    > enabled.
    >
    > Cc: Will Deacon <will.deacon@arm.com>
    > Cc: Dave Martin <dave.martin@arm.com>
    > Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
    > ---
    > arch/arm64/include/asm/cpufeature.h | 9 +++++++++
    > arch/arm64/kernel/cpufeature.c | 11 ++++++-----
    > 2 files changed, 15 insertions(+), 5 deletions(-)
    >
    > diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
    > index 7bb3fdec827e..71993dd4afae 100644
    > --- a/arch/arm64/include/asm/cpufeature.h
    > +++ b/arch/arm64/include/asm/cpufeature.h
    > @@ -223,6 +223,15 @@ extern struct arm64_ftr_reg arm64_ftr_reg_ctrel0;
    > ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU | \
    > ARM64_CPUCAP_PERMITTED_FOR_LATE_CPU)
    >
    > +/*
    > + * CPU feature detected at boot time, on one or more CPUs. A late CPU
    > + * is not allowed to have the capability when the system doesn't have it.
    > + * It is Ok for a late CPU to miss the feature.
    > + */
    > +#define ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE \
    > + (ARM64_CPUCAP_SCOPE_LOCAL_CPU | \
    > + ARM64_CPUCAP_OPTIONAL_FOR_LATE_CPU)
    > +
    > struct arm64_cpu_capabilities {
    > const char *desc;
    > u16 capability;
    > diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
    > index ecc87aa74c64..4a55492784b7 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,
    > - int __unused)
    > + int scope)
    > {
    > - u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
    >
    > /* Forced on command line? */
    > if (__kpti_forced) {
    > @@ -885,8 +884,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
    > }
    >
    > /* 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)
    > @@ -1008,7 +1006,10 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
    > {
    > .desc = "Kernel page table isolation (KPTI)",
    > .capability = ARM64_UNMAP_KERNEL_AT_EL0,
    > - .type = ARM64_CPUCAP_SYSTEM_FEATURE,
    > + .type = ARM64_CPUCAP_BOOT_RESTRICTED_CPU_LOCAL_FEATURE,
    > + .sys_reg = SYS_ID_AA64PFR0_EL1,
    > + .field_pos = ID_AA64PFR0_CSV3_SHIFT,
    > + .min_field_value = 1,
    > .matches = unmap_kernel_at_el0,

    Minor nit, but:

    Can we have a comment here to explain that .min_field_value is the
    minimum value that indicates that KPTI is _not_ required by this cpu?
    This is the opposite of the usual semantics for this field.

    Otherwise, this inversion of meaning is not obvious without digging into
    unmap_kernel_at_el0() and spotting the ! in !has_cpuid_feature().

    With that, or if this usage of !has_cpuid_feature() is already well-
    established so that a comment is deemed unnecessary:

    Reviewed-by: Dave Martin <Dave.Martin@arm.com>

    Cheers
    ---Dave
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