Re: [PATCH v5 03/10] arm64: add sysfs vulnerability show for meltdown

[Catalin Marinas]

On Fri, Mar 01, 2019 at 10:12:09AM -0600, Jeremy Linton wrote:
> On 3/1/19 1:11 AM, Andre Przywara wrote:
> > On 2/26/19 7:05 PM, Jeremy Linton wrote:
> > > Display the mitigation status if active, otherwise
> > > assume the cpu is safe unless it doesn't have CSV3
> > > and isn't in our whitelist.
> > > 
> > > Signed-off-by: Jeremy Linton <jeremy.linton@xxxxxxx>
> > > ---
> > >   arch/arm64/kernel/cpufeature.c | 47 ++++++++++++++++++++++++++--------
> > >   1 file changed, 37 insertions(+), 10 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kernel/cpufeature.c
> > > b/arch/arm64/kernel/cpufeature.c
> > > index f6d84e2c92fe..d31bd770acba 100644
> > > --- a/arch/arm64/kernel/cpufeature.c
> > > +++ b/arch/arm64/kernel/cpufeature.c
> > > @@ -944,7 +944,7 @@ has_useable_cnp(const struct
> > > arm64_cpu_capabilities *entry, int scope)
> > >       return has_cpuid_feature(entry, scope);
> > >   }
> > > -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > > +static bool __meltdown_safe = true;
> > >   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,
> > > @@ -963,6 +963,16 @@ static bool unmap_kernel_at_el0(const struct
> > > arm64_cpu_capabilities *entry,
> > >           { /* sentinel */ }
> > >       };
> > >       char const *str = "command line option";
> > > +    bool meltdown_safe;
> > > +
> > > +    meltdown_safe = is_midr_in_range_list(read_cpuid_id(),
> > > kpti_safe_list);
> > > +
> > > +    /* Defer to CPU feature registers */
> > > +    if (has_cpuid_feature(entry, scope))
> > > +        meltdown_safe = true;
> > > +
> > > +    if (!meltdown_safe)
> > > +        __meltdown_safe = false;
> > >       /*
> > >        * For reasons that aren't entirely clear, enabling KPTI on Cavium
> > > @@ -974,6 +984,11 @@ static bool unmap_kernel_at_el0(const struct
> > > arm64_cpu_capabilities *entry,
> > >           __kpti_forced = -1;
> > >       }
> > > +    if (!IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
> > > +        pr_info_once("kernel page table isolation disabled by
> > > CONFIG\n");
> > > +        return false;
> > > +    }
> > > +
> > >       /* Forced? */
> > >       if (__kpti_forced) {
> > >           pr_info_once("kernel page table isolation forced %s by %s\n",
> > > @@ -985,14 +1000,10 @@ static bool unmap_kernel_at_el0(const struct
> > > arm64_cpu_capabilities *entry,
> > >       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> > >           return kaslr_offset() > 0;
> > > -    /* Don't force KPTI for CPUs that are not vulnerable */
> > > -    if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
> > > -        return false;
> > > -
> > > -    /* Defer to CPU feature registers */
> > > -    return !has_cpuid_feature(entry, scope);
> > > +    return !meltdown_safe;
> > >   }
> > > +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > >   static void
> > >   kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> > >   {
> > > @@ -1022,6 +1033,13 @@ kpti_install_ng_mappings(const struct
> > > arm64_cpu_capabilities *__unused)
> > >       return;
> > >   }
> > > +#else
> > > +static void
> > > +kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> > > +{
> > > +}
> > > +#endif    /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> > > +
> > >   static int __init parse_kpti(char *str)
> > >   {
> > > @@ -1035,7 +1053,6 @@ static int __init parse_kpti(char *str)
> > >       return 0;
> > >   }
> > >   early_param("kpti", parse_kpti);
> > > -#endif    /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> > >   #ifdef CONFIG_ARM64_HW_AFDBM
> > >   static inline void __cpu_enable_hw_dbm(void)
> > > @@ -1286,7 +1303,6 @@ static const struct arm64_cpu_capabilities
> > > arm64_features[] = {
> > >           .field_pos = ID_AA64PFR0_EL0_SHIFT,
> > >           .min_field_value = ID_AA64PFR0_EL0_32BIT_64BIT,
> > >       },
> > > -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > >       {
> > >           .desc = "Kernel page table isolation (KPTI)",
> > >           .capability = ARM64_UNMAP_KERNEL_AT_EL0,
> > > @@ -1302,7 +1318,6 @@ static const struct arm64_cpu_capabilities
> > > arm64_features[] = {
> > >           .matches = unmap_kernel_at_el0,
> > >           .cpu_enable = kpti_install_ng_mappings,
> > >       },
> > > -#endif
> > >       {
> > >           /* FP/SIMD is not implemented */
> > >           .capability = ARM64_HAS_NO_FPSIMD,
> > > @@ -2063,3 +2078,15 @@ static int __init enable_mrs_emulation(void)
> > >   }
> > >   core_initcall(enable_mrs_emulation);
> > > +
> > > +ssize_t cpu_show_meltdown(struct device *dev, struct
> > > device_attribute *attr,
> > > +        char *buf)
> > > +{
> > > +    if (arm64_kernel_unmapped_at_el0())
> > > +        return sprintf(buf, "Mitigation: KPTI\n");
> > > +
> > > +    if (__meltdown_safe)
> > > +        return sprintf(buf, "Not affected\n");
> > 
> > Shall those two checks be swapped? So it doesn't report about a KPTI
> > mitigation if the CPU is safe, but we enable KPTI because of KASLR
> > having enabled it? Or is that a different knob?
> 
> Hmmm, I think having it this way reflects the fact that the machine is
> mitigated independent of whether it needed it. The force on case is similar.
> The machine may not have needed the mitigation but it was forced on.

So is this patchset about showing vulnerabilities _and_ mitigations or
just one of them?

-- 
Catalin