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

[Jeremy Linton]

Hi,

On 3/1/19 1:11 AM, Andre Przywara wrote:
> Hi,
>
> 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>
> > ---
> > Â arch/arm64/kernel/cpufeature.c | 47 ++++++++++++++++++++++++++--------
> > Â 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)
> > ÂÂÂÂÂ return has_cpuid_feature(entry, scope);
> > Â }
> > -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > +static bool __meltdown_safe = true;
> > Â static int __kpti_forced; /* 0: not forced, >0: forced on, <0: 
> > forced off */
> > Â 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,
> > ÂÂÂÂÂÂÂÂÂ { /* sentinel */ }
> > ÂÂÂÂÂ };
> > ÂÂÂÂÂ char const *str = "command line option";
> > +ÂÂÂ bool meltdown_safe;
> > +
> > +ÂÂÂ meltdown_safe = is_midr_in_range_list(read_cpuid_id(), 
> > kpti_safe_list);
> > +
> > +ÂÂÂ /* Defer to CPU feature registers */
> > +ÂÂÂ if (has_cpuid_feature(entry, scope))
> > +ÂÂÂÂÂÂÂ meltdown_safe = true;
> > +
> > +ÂÂÂ if (!meltdown_safe)
> > +ÂÂÂÂÂÂÂ __meltdown_safe = false;
> > ÂÂÂÂÂ /*
> > ÂÂÂÂÂÂ * 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,
> > ÂÂÂÂÂÂÂÂÂ __kpti_forced = -1;
> > ÂÂÂÂÂ }
> > +ÂÂÂ if (!IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
> > +ÂÂÂÂÂÂÂ pr_info_once("kernel page table isolation disabled by 
> > CONFIG\n");
> > +ÂÂÂÂÂÂÂ return false;
> > +ÂÂÂ }
> > +
> > ÂÂÂÂÂ /* Forced? */
> > ÂÂÂÂÂ if (__kpti_forced) {
> > ÂÂÂÂÂÂÂÂÂ 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,
> > ÂÂÂÂÂ if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> > ÂÂÂÂÂÂÂÂÂ return kaslr_offset() > 0;
> > -ÂÂÂ /* Don't force KPTI for CPUs that are not vulnerable */
> > -ÂÂÂ if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
> > -ÂÂÂÂÂÂÂ return false;
> > -
> > -ÂÂÂ /* Defer to CPU feature registers */
> > -ÂÂÂ return !has_cpuid_feature(entry, scope);
> > +ÂÂÂ return !meltdown_safe;
> > Â }
> > +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > Â static void
> > Â kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> > Â {
> > @@ -1022,6 +1033,13 @@ kpti_install_ng_mappings(const struct 
> > arm64_cpu_capabilities *__unused)
> > ÂÂÂÂÂ return;
> > Â }
> > +#else
> > +static void
> > +kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> > +{
> > +}
> > +#endifÂÂÂ /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> > +
> > Â static int __init parse_kpti(char *str)
> > Â {
> > @@ -1035,7 +1053,6 @@ static int __init parse_kpti(char *str)
> > ÂÂÂÂÂ return 0;
> > Â }
> > Â early_param("kpti", parse_kpti);
> > -#endifÂÂÂ /* CONFIG_UNMAP_KERNEL_AT_EL0 */
> > Â #ifdef CONFIG_ARM64_HW_AFDBM
> > Â static inline void __cpu_enable_hw_dbm(void)
> > @@ -1286,7 +1303,6 @@ static const struct arm64_cpu_capabilities 
> > arm64_features[] = {
> > ÂÂÂÂÂÂÂÂÂ .field_pos = ID_AA64PFR0_EL0_SHIFT,
> > ÂÂÂÂÂÂÂÂÂ .min_field_value = ID_AA64PFR0_EL0_32BIT_64BIT,
> > ÂÂÂÂÂ },
> > -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> > ÂÂÂÂÂ {
> > ÂÂÂÂÂÂÂÂÂ .desc = "Kernel page table isolation (KPTI)",
> > ÂÂÂÂÂÂÂÂÂ .capability = ARM64_UNMAP_KERNEL_AT_EL0,
> > @@ -1302,7 +1318,6 @@ static const struct arm64_cpu_capabilities 
> > arm64_features[] = {
> > ÂÂÂÂÂÂÂÂÂ .matches = unmap_kernel_at_el0,
> > ÂÂÂÂÂÂÂÂÂ .cpu_enable = kpti_install_ng_mappings,
> > ÂÂÂÂÂ },
> > -#endif
> > ÂÂÂÂÂ {
> > ÂÂÂÂÂÂÂÂÂ /* FP/SIMD is not implemented */
> > ÂÂÂÂÂÂÂÂÂ .capability = ARM64_HAS_NO_FPSIMD,
> > @@ -2063,3 +2078,15 @@ static int __init enable_mrs_emulation(void)
> > Â }
> > Â core_initcall(enable_mrs_emulation);
> > +
> > +ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute 
> > *attr,
> > +ÂÂÂÂÂÂÂ char *buf)
> > +{
> > +ÂÂÂ if (arm64_kernel_unmapped_at_el0())
> > +ÂÂÂÂÂÂÂ return sprintf(buf, "Mitigation: KPTI\n");
> > +
> > +ÂÂÂ if (__meltdown_safe)
> > +ÂÂÂÂÂÂÂ 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.


> Cheers,
> Andre.
>  
> > +
> > +ÂÂÂ return sprintf(buf, "Vulnerable\n");
> > +}