[RFC PATCH v3 05/10] x86/bugs: Use Virtual MSRs to request RRSBA_DIS_S

[Chao Gao]

From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>

On CPUs with RRSBA behavior a guest using retpoline mitigation could
become vulnerable to BHI. On such CPUs, when RSB underflows a RET could
take prediction from BTB. Although these predictions are limited to same
domain, they may be controllable from userspace using BHI.

Alderlake and newer CPUs have RRSBA_DIS_S knob in MSR_SPEC_CTRL to
disable RRSBA behavior. A guest migrating from older CPU may not be
aware of RRSBA_DIS_S. Use MSR_VIRTUAL_MITIGATION_CTRL to request VMM to
deploy RRSBA_DIS_S when retpoline mitigation is in use.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
---
 arch/x86/include/asm/msr-index.h | 6 ++++++
 arch/x86/kernel/cpu/bugs.c       | 7 +++++++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 18a4081bf5cb..469ab38c0ec8 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -1188,6 +1188,7 @@
 
 #define MSR_VIRTUAL_MITIGATION_ENUM		0x50000001
 #define MITI_ENUM_BHB_CLEAR_SEQ_S_SUPPORT	BIT(0)	/* VMM supports BHI_DIS_S */
+#define MITI_ENUM_RETPOLINE_S_SUPPORT		BIT(1)	/* VMM supports RRSBA_DIS_S */
 
 #define MSR_VIRTUAL_MITIGATION_CTRL		0x50000002
 #define MITI_CTRL_BHB_CLEAR_SEQ_S_USED_BIT	0	/*
@@ -1195,6 +1196,11 @@
 							 * BHI_DIS_S mitigation
 							 */
 #define MITI_CTRL_BHB_CLEAR_SEQ_S_USED		BIT(MITI_CTRL_BHB_CLEAR_SEQ_S_USED_BIT)
+#define MITI_CTRL_RETPOLINE_S_USED_BIT		1	/*
+							 * Request VMM to deploy
+							 * RRSBA_DIS_S mitigation
+							 */
+#define MITI_CTRL_RETPOLINE_S_USED		BIT(MITI_CTRL_RETPOLINE_S_USED_BIT)
 
 /* AMD-V MSRs */
 #define MSR_VM_CR                       0xc0010114
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index e74e4c51d387..766f4340eddf 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1704,6 +1704,13 @@ void virt_mitigation_ctrl_init(void)
 		else
 			msr_clear_bit(MSR_VIRTUAL_MITIGATION_CTRL, MITI_CTRL_BHB_CLEAR_SEQ_S_USED_BIT);
 	}
+	if (msr_mitigation_enum & MITI_ENUM_RETPOLINE_S_SUPPORT) {
+		/* When retpoline is being used, request RRSBA_DIS_S */
+		if (boot_cpu_has(X86_FEATURE_RETPOLINE))
+			msr_set_bit(MSR_VIRTUAL_MITIGATION_CTRL, MITI_CTRL_RETPOLINE_S_USED_BIT);
+		else
+			msr_clear_bit(MSR_VIRTUAL_MITIGATION_CTRL, MITI_CTRL_RETPOLINE_S_USED_BIT);
+	}
 }
 
 static void __init spectre_v2_select_mitigation(void)
-- 
2.39.3