Re: [RESEND RFC PATCH] x86/bugs: Add "unknown" reporting for MMIO Stale Data

[Dave Hansen]

On 7/14/22 18:30, Pawan Gupta wrote:
> Older CPUs beyond its Servicing period are not listed in the affected
> processor list for MMIO Stale Data vulnerabilities. These CPUs currently
> report "Not affected" in sysfs, which may not be correct.

I'd kinda like to remove the talk about the "servicing period" in this
patch.  First, it's a moving target.  CPUs can move in and out of their
servicing period as Intel changes its mind, or simply as time passes.

Intel could also totally choose to report a CPU as vulnerable *AND* have
it be outside its service period.  Or, some good Samaritan community
member might be able to test a crusty old CPU and determine if it's
vulnerable.

> diff --git a/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst b/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
> index 9393c50b5afc..55524e0798da 100644
> --- a/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
> +++ b/Documentation/admin-guide/hw-vuln/processor_mmio_stale_data.rst
> @@ -230,6 +230,9 @@ The possible values in this file are:
>       * - 'Mitigation: Clear CPU buffers'
>         - The processor is vulnerable and the CPU buffer clearing mitigation is
>           enabled.
> +     * - 'Unknown: CPU is beyond its Servicing period'
> +       - The processor vulnerability status is unknown because it is
> +	 out of Servicing period. Mitigation is not attempted.

Unknown: Processor vendor did not provide vulnerability status.

>  If the processor is vulnerable then the following information is appended to
>  the above information:
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 0dd04713434b..dd6e78d370bc 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -416,6 +416,7 @@ enum mmio_mitigations {
>  	MMIO_MITIGATION_OFF,
>  	MMIO_MITIGATION_UCODE_NEEDED,
>  	MMIO_MITIGATION_VERW,
> +	MMIO_MITIGATION_UNKNOWN,
>  };
>  
>  /* Default mitigation for Processor MMIO Stale Data vulnerabilities */
> @@ -426,12 +427,18 @@ static const char * const mmio_strings[] = {
>  	[MMIO_MITIGATION_OFF]		= "Vulnerable",
>  	[MMIO_MITIGATION_UCODE_NEEDED]	= "Vulnerable: Clear CPU buffers attempted, no microcode",
>  	[MMIO_MITIGATION_VERW]		= "Mitigation: Clear CPU buffers",
> +	[MMIO_MITIGATION_UNKNOWN]	= "Unknown: CPU is beyond its servicing period",
>  };

Let's just say:

	Unknown: no mitigations

or even just: "Unknown"

>  static void __init mmio_select_mitigation(void)
>  {
>  	u64 ia32_cap;
>  
> +	if (mmio_stale_data_unknown()) {
> +		mmio_mitigation = MMIO_MITIGATION_UNKNOWN;
> +		return;
> +	}
> +
>  	if (!boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA) ||
>  	    cpu_mitigations_off()) {
>  		mmio_mitigation = MMIO_MITIGATION_OFF;
> @@ -1638,6 +1645,7 @@ void cpu_bugs_smt_update(void)
>  			pr_warn_once(MMIO_MSG_SMT);
>  		break;
>  	case MMIO_MITIGATION_OFF:
> +	case MMIO_MITIGATION_UNKNOWN:
>  		break;
>  	}
>  
> @@ -2235,7 +2243,8 @@ static ssize_t tsx_async_abort_show_state(char *buf)
>  
>  static ssize_t mmio_stale_data_show_state(char *buf)
>  {
> -	if (mmio_mitigation == MMIO_MITIGATION_OFF)
> +	if (mmio_mitigation == MMIO_MITIGATION_OFF ||
> +	    mmio_mitigation == MMIO_MITIGATION_UNKNOWN)
>  		return sysfs_emit(buf, "%s\n", mmio_strings[mmio_mitigation]);
>  
>  	if (boot_cpu_has(X86_FEATURE_HYPERVISOR)) {
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index 736262a76a12..82088410870e 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -1286,6 +1286,22 @@ static bool arch_cap_mmio_immune(u64 ia32_cap)
>  		ia32_cap & ARCH_CAP_SBDR_SSDP_NO);
>  }
>  
> +bool __init mmio_stale_data_unknown(void)
> +{
> +	u64 ia32_cap = x86_read_arch_cap_msr();
> +
> +	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL)
> +		return false;

Let's say why Intel is the special snowflake.  Maybe:

	/*
	 * Intel does not document vulnerability information for old
	 * CPUs.  This means that only Intel CPUs can have unknown
	 * vulnerability state.
	 */

> +	/*
> +	 * CPU vulnerability is unknown when, hardware doesn't set the
> +	 * immunity bits and CPU is not in the known affected list.
> +	 */
> +	if (!cpu_matches(cpu_vuln_blacklist, MMIO) &&
> +	    !arch_cap_mmio_immune(ia32_cap))
> +		return true;
> +	return false;
> +}
> +
>  static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
>  {
>  	u64 ia32_cap = x86_read_arch_cap_msr();
> @@ -1349,14 +1365,8 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
>  	    cpu_matches(cpu_vuln_blacklist, SRBDS | MMIO_SBDS))
>  		    setup_force_cpu_bug(X86_BUG_SRBDS);
>  
> -	/*
> -	 * Processor MMIO Stale Data bug enumeration
> -	 *
> -	 * Affected CPU list is generally enough to enumerate the vulnerability,
> -	 * but for virtualization case check for ARCH_CAP MSR bits also, VMM may
> -	 * not want the guest to enumerate the bug.
> -	 */
> -	if (cpu_matches(cpu_vuln_blacklist, MMIO) &&
> +	 /* Processor MMIO Stale Data bug enumeration */
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
>  	    !arch_cap_mmio_immune(ia32_cap))
>  		setup_force_cpu_bug(X86_BUG_MMIO_STALE_DATA);

Yeah, this is all looking a little clunky.

Maybe we just need a third state of cpu_has_bug() for all this and we
shouldn't try cramming it in the MMIO-specific code and diluting the
specificity of boot_cpu_has_bug().

Then the selection logic becomes simple:

	if (!arch_cap_mmio_immune(ia32_cap))) {
		if (cpu_matches(cpu_vuln_blacklist, MMIO))
			setup_force_cpu_bug(X86_BUG_MMIO_STALE_DATA);
		else if (x86_vendor == X86_VENDOR_INTEL)
			setup_force_unknown_bug(X86_BUG_MMIO...);
	}

... and then spit out the "Unknown" in the common code, just like the
treatment "Not affected" gets.

static ssize_t cpu_show_common(...)
{
        if (!boot_cpu_has_bug(bug))
                return sprintf(buf, "Not affected\n");
+
+       if (!boot_cpu_unknown_bug(bug))
+               return sprintf(buf, "Unknown\n");

Thoughts?