Re: [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel processors

Dave Hansen
2018-01-23T18:40:33Z

On 01/23/2018 08:52 AM, David Woodhouse wrote:
> When they advertise the IA32_ARCH_CAPABILITIES MSR and it has the RDCL_NO
> bit set, they don't need KPTI either.
>
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
> arch/x86/kernel/cpu/common.c | 10 ++++++++--
> 1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index e5d66e9..c05d0fe 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -900,8 +900,14 @@ static void __init early_identify_cpu(struct cpuinfo_x86 *c)
>
> setup_force_cpu_cap(X86_FEATURE_ALWAYS);
>
> - if (c->x86_vendor != X86_VENDOR_AMD)
> - setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
> + if (c->x86_vendor != X86_VENDOR_AMD) {
> + u64 ia32_cap = 0;
> +
> + if (cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
> + rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
> + if (!(ia32_cap & ARCH_CAP_RDCL_NO))
> + setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
> + }

I'd really rather we break this out into a nice, linear set of
true/false conditions.

bool early_cpu_vulnerable_meltdown(struct cpuinfo_x86 *c)
{
u64 ia32_cap = 0;

/* AMD processors are not subject to Meltdown exploit: */
if (c->x86_vendor == X86_VENDOR_AMD)
return false;

/* Assume all remaining CPUs not enumerating are vulnerable: */
if (!cpu_has(c, X86_FEATURE_ARCH_CAPABILITIES))
return true;

/*
* Does the CPU explicitly enumerate that it is not vulnerable
* to Rogue Data Cache Load (aka Meltdown)?
*/
rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
if (ia32_cap & ARCH_CAP_RDCL_NO)
return false;

/* Assume everything else is vulnerable */
return true;
}

Then we get a nice:

if (early_cpu_vulnerable_meltdown(c))
setup_force_cpu_bug(X86_BUG_CPU_MELTDOWN);
setup_force_cpu_bug(X86_BUG_SPECTRE_V1);
setup_force_cpu_bug(X86_BUG_SPECTRE_V2);

Which clearly shows that Meltdown is special.