x86/speculation: Identify processors vulnerable to SMT RSB predictions

Certain AMD processors are vulnerable to a cross-thread return address
predictions bug. When running in SMT mode and one of the sibling threads
transitions out of C0 state, the other sibling thread could use return
target predictions from the sibling thread that transitioned out of C0.

The Spectre v2 mitigations cover the Linux kernel, as it fills the RSB
when context switching to the idle thread. However, KVM allows a VMM to
prevent exiting guest mode when transitioning out of C0. A guest could
act maliciously in this situation, so create a new x86 BUG that can be
used to detect if the processor is vulnerable.
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <91cec885656ca1fcd4f0185ce403a53dd9edecb7.1675956146.git.thomas.lendacky@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/cpufeatures.h

@@ -466,5 +466,6 @@
 #define X86_BUG_MMIO_UNKNOWN		X86_BUG(26) /* CPU is too old and its MMIO Stale Data status is unknown */
 #define X86_BUG_RETBLEED		X86_BUG(27) /* CPU is affected by RETBleed */
 #define X86_BUG_EIBRS_PBRSB		X86_BUG(28) /* EIBRS is vulnerable to Post Barrier RSB Predictions */
+#define X86_BUG_SMT_RSB			X86_BUG(29) /* CPU is vulnerable to Cross-Thread Return Address Predictions */
 
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/kernel/cpu/common.c

@@ -1256,6 +1256,8 @@ static const __initconst struct x86_cpu_id cpu_vuln_whitelist[] = {
 #define MMIO_SBDS	BIT(2)
 /* CPU is affected by RETbleed, speculating where you would not expect it */
 #define RETBLEED	BIT(3)
+/* CPU is affected by SMT (cross-thread) return predictions */
+#define SMT_RSB		BIT(4)
 
 static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
 	VULNBL_INTEL_STEPPINGS(IVYBRIDGE,	X86_STEPPING_ANY,		SRBDS),
@@ -1287,8 +1289,8 @@ static const struct x86_cpu_id cpu_vuln_blacklist[] __initconst = {
 
 	VULNBL_AMD(0x15, RETBLEED),
 	VULNBL_AMD(0x16, RETBLEED),
-	VULNBL_AMD(0x17, RETBLEED),
-	VULNBL_HYGON(0x18, RETBLEED),
+	VULNBL_AMD(0x17, RETBLEED | SMT_RSB),
+	VULNBL_HYGON(0x18, RETBLEED | SMT_RSB),
 	{}
 };
 
@@ -1406,6 +1408,9 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	    !(ia32_cap & ARCH_CAP_PBRSB_NO))
 		setup_force_cpu_bug(X86_BUG_EIBRS_PBRSB);
 
+	if (cpu_matches(cpu_vuln_blacklist, SMT_RSB))
+		setup_force_cpu_bug(X86_BUG_SMT_RSB);
+
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;