[PATCH] x86/sev: Harden #VC insn emulation somewhat

[Borislav Petkov]

From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>

Compare the opcode bytes at rIP for each #VC exit reason to verify the
instruction which raised the #VC exception is actually the right one.

Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
---
 arch/x86/boot/compressed/sev.c |   4 ++
 arch/x86/kernel/sev-shared.c   | 102 ++++++++++++++++++++++++++++++++-
 arch/x86/kernel/sev.c          |   9 ++-
 3 files changed, 108 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/sev.c b/arch/x86/boot/compressed/sev.c
index 454acd7a2daf..073291832f44 100644
--- a/arch/x86/boot/compressed/sev.c
+++ b/arch/x86/boot/compressed/sev.c
@@ -304,6 +304,10 @@ void do_boot_stage2_vc(struct pt_regs *regs, unsigned long exit_code)
 	if (result != ES_OK)
 		goto finish;
 
+	result = vc_check_opcode_bytes(&ctxt, exit_code);
+	if (result != ES_OK)
+		goto finish;
+
 	switch (exit_code) {
 	case SVM_EXIT_RDTSC:
 	case SVM_EXIT_RDTSCP:
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index ccb0915e84e1..ec17931bf3cd 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -10,11 +10,15 @@
  */
 
 #ifndef __BOOT_COMPRESSED
-#define error(v)	pr_err(v)
-#define has_cpuflag(f)	boot_cpu_has(f)
+#define error(v)			pr_err(v)
+#define has_cpuflag(f)			boot_cpu_has(f)
+#define sev_printk(fmt, ...)		printk(fmt, ##__VA_ARGS__)
+#define sev_printk_rtl(fmt, ...)	printk_ratelimited(fmt, ##__VA_ARGS__)
 #else
 #undef WARN
 #define WARN(condition, format...) (!!(condition))
+#define sev_printk(fmt, ...)
+#define sev_printk_rtl(fmt, ...)
 #endif
 
 /* I/O parameters for CPUID-related helpers */
@@ -574,6 +578,7 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 {
 	unsigned int subfn = lower_bits(regs->cx, 32);
 	unsigned int fn = lower_bits(regs->ax, 32);
+	u16 opcode = *(unsigned short *)regs->ip;
 	struct cpuid_leaf leaf;
 	int ret;
 
@@ -581,6 +586,10 @@ void __init do_vc_no_ghcb(struct pt_regs *regs, unsigned long exit_code)
 	if (exit_code != SVM_EXIT_CPUID)
 		goto fail;
 
+	/* Is it really a CPUID insn? */
+	if (opcode != 0xa20f)
+		goto fail;
+
 	leaf.fn = fn;
 	leaf.subfn = subfn;
 
@@ -1170,3 +1179,92 @@ static int vmgexit_psc(struct ghcb *ghcb, struct snp_psc_desc *desc)
 out:
 	return ret;
 }
+
+static enum es_result vc_check_opcode_bytes(struct es_em_ctxt *ctxt,
+					    unsigned long exit_code)
+{
+	unsigned int opcode = (unsigned int)ctxt->insn.opcode.value;
+	u8 modrm = ctxt->insn.modrm.value;
+
+	switch (exit_code) {
+
+	case SVM_EXIT_IOIO:
+	case SVM_EXIT_NPF:
+		/* handled separately */
+		return ES_OK;
+
+	case SVM_EXIT_CPUID:
+		if (opcode == 0xa20f)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_INVD:
+		sev_printk_rtl(KERN_ERR "#VC exception for INVD??? Seriously???\n");
+		return ES_UNSUPPORTED;
+		break;
+
+	case SVM_EXIT_MONITOR:
+		if (opcode == 0x010f && modrm == 0xc8)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_MWAIT:
+		if (opcode == 0x010f && modrm == 0xc9)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_MSR:
+		/* RDMSR */
+		if (opcode == 0x320f ||
+		/* WRMSR */
+		    opcode == 0x300f)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_RDPMC:
+		if (opcode == 0x330f)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_RDTSC:
+		if (opcode == 0x310f)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_RDTSCP:
+		if (opcode == 0x010f && modrm == 0xf9)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_READ_DR7:
+		if (opcode == 0x210f &&
+		    X86_MODRM_REG(ctxt->insn.modrm.value) == 7)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_VMMCALL:
+		if (opcode == 0x010f && modrm == 0xd9)
+			return ES_OK;
+
+		break;
+
+	case SVM_EXIT_WRITE_DR7:
+		if (opcode == 0x230f &&
+		    X86_MODRM_REG(ctxt->insn.modrm.value) == 7)
+			return ES_OK;
+		break;
+
+	case SVM_EXIT_WBINVD:
+		if (opcode == 0x90f)
+			return ES_OK;
+		break;
+
+	default:
+		break;
+	}
+
+	sev_printk(KERN_ERR "Wrong/unhandled opcode bytes: 0x%x, exit_code: 0x%lx, rIP: 0x%lx\n",
+		   opcode, exit_code, ctxt->regs->ip);
+
+	return ES_UNSUPPORTED;
+}
diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index c67285824e82..541a1f87b4f1 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c
@@ -1752,7 +1752,10 @@ static enum es_result vc_handle_exitcode(struct es_em_ctxt *ctxt,
 					 struct ghcb *ghcb,
 					 unsigned long exit_code)
 {
-	enum es_result result;
+	enum es_result result = vc_check_opcode_bytes(ctxt, exit_code);
+
+	if (result != ES_OK)
+		return result;
 
 	switch (exit_code) {
 	case SVM_EXIT_READ_DR7:
@@ -1771,10 +1774,6 @@ static enum es_result vc_handle_exitcode(struct es_em_ctxt *ctxt,
 	case SVM_EXIT_RDPMC:
 		result = vc_handle_rdpmc(ghcb, ctxt);
 		break;
-	case SVM_EXIT_INVD:
-		pr_err_ratelimited("#VC exception for INVD??? Seriously???\n");
-		result = ES_UNSUPPORTED;
-		break;
 	case SVM_EXIT_CPUID:
 		result = vc_handle_cpuid(ghcb, ctxt);
 		break;
-- 
2.42.0.rc0.25.ga82fb66fed25