Messages in this thread | | | Subject | Re: [PATCH v3 4/7] arm64: add sysfs vulnerability show for meltdown | From | Jeremy Linton <> | Date | Thu, 10 Jan 2019 08:10:43 -0600 |
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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.
> > 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, >
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