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SubjectRe: [PATCH] x86/spectre: Expand test for vulnerability to empty RSB exploits
On Tue, Aug 07, 2018 at 03:25:35PM -0700, Jim Mattson wrote:
> Skylake-era Intel CPUs are vulnerable to exploits of empty RSB
> conditions. On hardware, platform vulnerability can be determined
> simply by checking the processor's DisplayModel/DisplayFamily
> signature. However, when running in a VM, the operating system should
> also query IA32_ARCH_CAPABILITIES.RSBA[bit 2], a synthetic bit that
> can be set by a hypervisor to indicate that the VM might run on a
> vulnerable physical processor, regardless of the
> DisplayModel/DisplayFamily reported by CPUID.
>
> Note that IA32_ARCH_CAPABILITIES.RSBA[bit 2] is always clear on
> hardware, so the DisplayModel/DisplayFamily check is still required.
>
> For all of the details, see the Intel white paper, "Retpoline: A
> Branch Target Injection Mitigation" (document number 337131-001),
> section 5.3: Virtual Machine CPU Identification.
>
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Thank you as it saves me from doing this :-)
> ---
> arch/x86/include/asm/msr-index.h | 1 +
> arch/x86/kernel/cpu/bugs.c | 14 +++++++++++++-
> 2 files changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 68b2c3150de1..f37ec58c4e04 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -70,6 +70,7 @@
> #define MSR_IA32_ARCH_CAPABILITIES 0x0000010a
> #define ARCH_CAP_RDCL_NO (1 << 0) /* Not susceptible to Meltdown */
> #define ARCH_CAP_IBRS_ALL (1 << 1) /* Enhanced IBRS support */
> +#define ARCH_CAP_RSBA (1 << 2) /* Vulnerable to empty RSB */
> #define ARCH_CAP_SSB_NO (1 << 4) /*
> * Not susceptible to Speculative Store Bypass
> * attack, so no Speculative Store Bypass
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 5c0ea39311fe..b6fe335746a4 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -330,6 +330,18 @@ static bool __init is_skylake_era(void)
> return false;
> }
>
> +/* Check for vulnerability to exploits of empty RSB conditions */
> +static bool __init is_vulnerable_to_empty_rsb(void)
> +{
> + u64 ia32_cap = 0;
> +
> + if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
> + rdmsrl(MSR_IA32_ARCH_CAPABILITIES, ia32_cap);
> +
> + return (ia32_cap & ARCH_CAP_RSBA) || is_skylake_era();
> +}
> +
> +
> static void __init spectre_v2_select_mitigation(void)
> {
> enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
> @@ -402,7 +414,7 @@ static void __init spectre_v2_select_mitigation(void)
> * switch is required.
> */
> if ((!boot_cpu_has(X86_FEATURE_PTI) &&
> - !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
> + !boot_cpu_has(X86_FEATURE_SMEP)) || is_vulnerable_to_empty_rsb()) {
> setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> pr_info("Spectre v2 mitigation: Filling RSB on context switch\n");
> }
> --
> 2.18.0.597.ga71716f1ad-goog
>

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