Messages in this thread | | | Subject | Re: [PATCH v3 4/7] arm64: add sysfs vulnerability show for meltdown | From | Julien Thierry <> | Date | Thu, 10 Jan 2019 14:16:01 +0000 |
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On 10/01/2019 14:10, Jeremy Linton wrote: > Hi Julien, > > On 01/10/2019 03:23 AM, Julien Thierry wrote: >> Hi Jeremy, >> >> On 09/01/2019 23:55, 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@arm.com> >>> --- >>> arch/arm64/kernel/cpufeature.c | 32 +++++++++++++++++++++++++++----- >>> 1 file changed, 27 insertions(+), 5 deletions(-) >>> >>> diff --git a/arch/arm64/kernel/cpufeature.c >>> b/arch/arm64/kernel/cpufeature.c >>> index ab784d7a0083..ef7bbc49ef78 100644 >>> --- a/arch/arm64/kernel/cpufeature.c >>> +++ b/arch/arm64/kernel/cpufeature.c >>> @@ -944,8 +944,12 @@ 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 */ >>> +static bool __meltdown_safe = true; >>> + >>> #ifdef CONFIG_UNMAP_KERNEL_AT_EL0 >>> static int __kpti_forced; /* 0: not forced, >0: forced on, <0: >>> forced off */ >>> +extern uint arm64_requested_vuln_attrs; >>> static bool is_cpu_meltdown_safe(void) >>> { >>> @@ -972,6 +976,14 @@ static bool unmap_kernel_at_el0(const struct >>> arm64_cpu_capabilities *entry, >>> { >>> char const *str = "command line option"; >>> + bool meltdown_safe = is_cpu_meltdown_safe() || >>> + has_cpuid_feature(entry, scope); >>> + >>> + if (!meltdown_safe) >>> + __meltdown_safe = false; >>> + >>> + arm64_requested_vuln_attrs |= VULN_MELTDOWN; >>> + >>> /* >>> * For reasons that aren't entirely clear, enabling KPTI on Cavium >>> * ThunderX leads to apparent I-cache corruption of kernel >>> text, which >>> @@ -993,11 +1005,7 @@ static bool unmap_kernel_at_el0(const struct >>> arm64_cpu_capabilities *entry, >>> if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) >>> return true; >>> - if (is_cpu_meltdown_safe()) >>> - return false; >>> - >>> - /* Defer to CPU feature registers */ >>> - return !has_cpuid_feature(entry, scope); >>> + return !meltdown_safe; >>> } >>> static void >>> @@ -2065,3 +2073,17 @@ 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) >>> +{ >>> + if (arm64_kernel_unmapped_at_el0()) >>> + return sprintf(buf, "Mitigation: KPTI\n"); >>> + >>> + if (__meltdown_safe) >>> + return sprintf(buf, "Not affected\n"); >> >> An issue I see is that we don't even bother to check it that CPUs are >> meltdown safe if CONFIG_UNMAP_KERNEL_AT_EL0 is not defined but here >> we'll advertise that the system is meltdown safe. > > That check isn't necessary anymore because the sysfs attribute is only > populated if unmap_kernel_at_el0() runs (assuming I haven't messed > something up). That was Dave/Will's suggestions in the last thread about > how to handle this case. >
Oh right, I missed that bit. Sorry for the noise.
>> >> I think that checking whether we know that CPUs are meltdown safe should >> be separated from whether mitigation is applied. >> >> Someone who knows thinks their CPUs are in the white list might want to >> compile out code that does the kpti, but it would be good to give them a >> proper diagnostic whether they were wrong or not. >> >> Cheers, >> >
-- Julien Thierry
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