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SubjectRe: [PATCH v4 07/12] arm64: add sysfs vulnerability show for meltdown
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Hi,

On 01/31/2019 11:54 AM, Andre Przywara wrote:
> On Fri, 25 Jan 2019 12:07:06 -0600
> Jeremy Linton <jeremy.linton@arm.com> wrote:
>
> Hi,
>
>> 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@arm.com>
>> ---
>> arch/arm64/kernel/cpufeature.c | 33 +++++++++++++++++++++++++++------
>> 1 file changed, 27 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c
>> b/arch/arm64/kernel/cpufeature.c
>> index a9e18b9cdc1e..624dfe0b5cdd 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -944,6 +944,8 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
>> return has_cpuid_feature(entry, scope);
>> }
>>
>> +/* default value is invalid until unmap_kernel_at_el0() runs */
>
> Shall we somehow enforce this? For instance by making __meltdown_safe
> an enum, initialised to UNKNOWN?

Hehe, well I think people complained about my "UNKNOWN" enum. But, in
the end this version is trying to make it clear we shouldn't have any
unknown states remaining.

> Then bail out with a BUG_ON or WARN_ON in the sysfs code?

AFAIK, it shouldn't be possible to actually run the sysfs code before
this gets initialized. So, the comment is just making it clear/forcing
the understanding of that.


>
> I just want to avoid to accidentally report "safe" when we actually
> aren't.
>
>> +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,
>> @@ -962,6 +964,16 @@ 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
>> @@ -984,12 +996,7 @@ 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;
>> }
>>
>> static void
>> @@ -2055,3 +2062,17 @@ static int __init enable_mrs_emulation(void)
>> }
>>
>> core_initcall(enable_mrs_emulation);
>> +
>> +#ifdef CONFIG_GENERIC_CPU_VULNERABILITIES
>> +ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
>> + char *buf)
>
> w/s issue.
>
> Cheers,
> Andre.
>
>> +{
>> + if (arm64_kernel_unmapped_at_el0())
>> + return sprintf(buf, "Mitigation: KPTI\n");
>> +
>> + if (__meltdown_safe)
>> + return sprintf(buf, "Not affected\n");
>> +
>> + return sprintf(buf, "Vulnerable\n");
>> +}
>> +#endif
>

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