[PATCH] x86/spectre: Expand test for vulnerability to empty RSB exploits

[Jim Mattson <jmattson@google.com>]

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>
---
 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