Hi Jeremy,
On 06/12/2018 23:44, Jeremy Linton wrote:
> Add a simple state machine which will track whether
> all the online cores in a machine are vulnerable.
>
> Once that is done we have a fairly authoritative view
> of the machine vulnerability, which allows us to make a
> judgment about machine safety if it hasn't been mitigated.
>
> Signed-off-by: Jeremy Linton <[email protected]>
> ---
> arch/arm64/kernel/cpufeature.c | 31 ++++++++++++++++++++++++++-----
> 1 file changed, 26 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 242898395f68..bea9adfef7fa 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -905,6 +905,8 @@ has_useable_cnp(const struct arm64_cpu_capabilities
> *entry, int scope)
> return has_cpuid_feature(entry, scope);
> }
>
> +static enum { A64_MELT_UNSET, A64_MELT_SAFE, A64_MELT_UNKN } __meltdown_safe
> = A64_MELT_UNSET;
> +
I'm wondering, do we really need that tri state?
Can't we consider that we are safe an move to unsafe/unkown if any cpu
during bring up is not in the safe list?
The only user of this is cpu_show_meltdown, but I don't imagine it'll
get called before unmap_kernel_at_el0() is called for the boot CPU which
should initialise that state.
Or is there another reason for having that UNSET state?
Thanks,
> #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
>
> @@ -928,6 +930,15 @@ 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);
> +
> + /* Only safe if all booted cores are known safe */
> + if (meltdown_safe && __meltdown_safe == A64_MELT_UNSET)
> + __meltdown_safe = A64_MELT_SAFE;
> + else if (!meltdown_safe)
> + __meltdown_safe = A64_MELT_UNKN;
> +
> /*
> * For reasons that aren't entirely clear, enabling KPTI on Cavium
> * ThunderX leads to apparent I-cache corruption of kernel text, which
> @@ -949,11 +960,7 @@ 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
> @@ -1920,3 +1927,17 @@ 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 == A64_MELT_SAFE)
> + return sprintf(buf, "Not affected\n");
> +
> + return sprintf(buf, "Unknown\n");
> +}
> +#endif
>
--
Julien Thierry
