[PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD

["Yang, Sheng" <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

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From 9743b5299bae1779c2b893cbeb86122bcccb9b2d Mon Sep 17 00:00:00 2001
From: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Date: Wed, 2 Jan 2008 14:49:22 +0800
Subject: [PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD

When executing a test program called "crashme", we found the KVM guest cannot 
survived more than ten seconds, then encounterd kernel panic. The basic 
concept of "crashme" is graduating random assembly code and trying to execute 
them in a fork process.

After some fix on emulator valid judgment, we found it's hard to get the 
current emulator handle the invalid instructions correctly, for the #UD trap 
for hypercall patching caused troubles. The problem is, if the opcode itself 
was OK, but combination of opcode and modrm_reg was invalid, and one operand 
of the opcode was memory(SrcMem or DstMem), emulator would fetched the memory 
operand first rather than judged the validity, and may encounter error there. 
For example, ".byte 0xfe, 0x34, 0xcd" got this trouble.

In the patch, we simply check that if the invalid opcode isn't vmcall/vmmcall, 
then return from emulate_instruction() and inject a #UD to guest. With the 
patch, the guest had been run for more than 12 hours.

Signed-off-by: Feng (Eric) Liu <eric.e.liu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Signed-off-by: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
---
 arch/x86/kvm/svm.c         |    2 +-
 arch/x86/kvm/vmx.c         |    2 +-
 arch/x86/kvm/x86.c         |   18 +++++++++++++++---
 include/asm-x86/kvm_host.h |    4 +++-
 4 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 11e5baf..2da9e11 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -942,7 +942,7 @@ static int ud_interception(struct vcpu_svm *svm, struct 
kvm_run *kvm_run)
 {
 	int er;
 
-	er = emulate_instruction(&svm->vcpu, kvm_run, 0, 0, 0);
+	er = emulate_instruction(&svm->vcpu, kvm_run, 0, 0, EMULTYPE_TRAP_UD);
 	if (er != EMULATE_DONE)
 		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
 	return 1;
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 4741806..5346e42 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1862,7 +1862,7 @@ static int handle_exception(struct kvm_vcpu *vcpu, 
struct kvm_run *kvm_run)
 	}
 
 	if (is_invalid_opcode(intr_info)) {
-		er = emulate_instruction(vcpu, kvm_run, 0, 0, 0);
+		er = emulate_instruction(vcpu, kvm_run, 0, 0, EMULTYPE_TRAP_UD);
 		if (er != EMULATE_DONE)
 			kvm_queue_exception(vcpu, UD_VECTOR);
 		return 1;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c5b4825..2a659ce 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1840,9 +1840,10 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 			struct kvm_run *run,
 			unsigned long cr2,
 			u16 error_code,
-			int no_decode)
+			int emulation_type)
 {
 	int r;
+	struct decode_cache *c;
 
 	vcpu->arch.mmio_fault_cr2 = cr2;
 	kvm_x86_ops->cache_regs(vcpu);
@@ -1850,7 +1851,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 	vcpu->mmio_is_write = 0;
 	vcpu->arch.pio.string = 0;
 
-	if (!no_decode) {
+	if (!(emulation_type & EMULTYPE_NO_DECODE)) {
 		int cs_db, cs_l;
 		kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 
@@ -1884,6 +1885,16 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 					get_segment_base(vcpu, VCPU_SREG_FS);
 
 		r = x86_decode_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
+
+		/* Reject the instructions other than VMCALL/VMMCALL when
+		 * try to emulate invalid opcode */
+		c = &vcpu->arch.emulate_ctxt.decode;
+		if ((emulation_type & EMULTYPE_TRAP_UD) &&
+		    (!(c->twobyte && c->b == 0x01 &&
+		      (c->modrm_reg == 0 || c->modrm_reg == 3) &&
+		       c->modrm_mod == 3 && c->modrm_rm == 1)))
+			return EMULATE_FAIL;
+
 		++vcpu->stat.insn_emulation;
 		if (r)  {
 			++vcpu->stat.insn_emulation_fail;
@@ -2639,7 +2650,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, 
struct kvm_run *kvm_run)
 		vcpu->mmio_read_completed = 1;
 		vcpu->mmio_needed = 0;
 		r = emulate_instruction(vcpu, kvm_run,
-					vcpu->arch.mmio_fault_cr2, 0, 1);
+					vcpu->arch.mmio_fault_cr2, 0,
+					EMULTYPE_NO_DECODE);
 		if (r == EMULATE_DO_MMIO) {
 			/*
 			 * Read-modify-write.  Back to userspace.
diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 20597bc..4702b04 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -415,8 +415,10 @@ enum emulation_result {
 	EMULATE_FAIL,         /* can't emulate this instruction */
 };
 
+#define EMULTYPE_NO_DECODE	    (1 << 0)
+#define EMULTYPE_TRAP_UD	    (1 << 1)
 int emulate_instruction(struct kvm_vcpu *vcpu, struct kvm_run *run,
-			unsigned long cr2, u16 error_code, int no_decode);
+			unsigned long cr2, u16 error_code, int emulation_type);
 void kvm_report_emulation_failure(struct kvm_vcpu *cvpu, const char 
*context);
 void realmode_lgdt(struct kvm_vcpu *vcpu, u16 size, unsigned long address);
 void realmode_lidt(struct kvm_vcpu *vcpu, u16 size, unsigned long address);
-- 
1.5.3.4


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From 9743b5299bae1779c2b893cbeb86122bcccb9b2d Mon Sep 17 00:00:00 2001
From: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Date: Wed, 2 Jan 2008 14:49:22 +0800
Subject: [PATCH] KVM: emulator: Only allow VMCALL/VMMCALL trapped by #UD

When executing a test program called "crashme", we found the KVM guest cannot survived more than ten seconds, then encounterd kernel panic. The basic concept of "crashme" is graduating random assembly code and trying to execute them.

After some fix on emulator valid judgment, we found it's hard to get the current emulator handle the invalid instructions correctly, for the #UD trap for hypercall patching caused troubles. The problem is, if the opcode itself was OK, but combination of opcode and modrm_reg was invalid, and one operand of the opcode was memory(SrcMem or DstMem), emulator would fetched the memory operand first rather than judged the validity, and may encounter error there. For example, ".byte 0xfe, 0x34, 0xcd" got this trouble.

In the patch, we simply checked that if the invalid opcode wasn't vmcall/vmmcall, then return from emulate_instruction() and inject a #UD to guest. With the patch, the guest had been running for more than 12 hours.

Signed-off-by: Feng (Eric) Liu <eric.e.liu-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Signed-off-by: Sheng Yang <sheng.yang-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
---
 arch/x86/kvm/svm.c         |    2 +-
 arch/x86/kvm/vmx.c         |    2 +-
 arch/x86/kvm/x86.c         |   18 +++++++++++++++---
 include/asm-x86/kvm_host.h |    4 +++-
 4 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 11e5baf..2da9e11 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -942,7 +942,7 @@ static int ud_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)
 {
 	int er;
 
-	er = emulate_instruction(&svm->vcpu, kvm_run, 0, 0, 0);
+	er = emulate_instruction(&svm->vcpu, kvm_run, 0, 0, EMULTYPE_TRAP_UD);
 	if (er != EMULATE_DONE)
 		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
 	return 1;
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 4741806..5346e42 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1862,7 +1862,7 @@ static int handle_exception(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	}
 
 	if (is_invalid_opcode(intr_info)) {
-		er = emulate_instruction(vcpu, kvm_run, 0, 0, 0);
+		er = emulate_instruction(vcpu, kvm_run, 0, 0, EMULTYPE_TRAP_UD);
 		if (er != EMULATE_DONE)
 			kvm_queue_exception(vcpu, UD_VECTOR);
 		return 1;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c5b4825..2a659ce 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1840,9 +1840,10 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 			struct kvm_run *run,
 			unsigned long cr2,
 			u16 error_code,
-			int no_decode)
+			int emulation_type)
 {
 	int r;
+	struct decode_cache *c;
 
 	vcpu->arch.mmio_fault_cr2 = cr2;
 	kvm_x86_ops->cache_regs(vcpu);
@@ -1850,7 +1851,7 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 	vcpu->mmio_is_write = 0;
 	vcpu->arch.pio.string = 0;
 
-	if (!no_decode) {
+	if (!(emulation_type & EMULTYPE_NO_DECODE)) {
 		int cs_db, cs_l;
 		kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 
@@ -1884,6 +1885,16 @@ int emulate_instruction(struct kvm_vcpu *vcpu,
 					get_segment_base(vcpu, VCPU_SREG_FS);
 
 		r = x86_decode_insn(&vcpu->arch.emulate_ctxt, &emulate_ops);
+
+		/* Reject the instructions other than VMCALL/VMMCALL when
+		 * try to emulate invalid opcode */
+		c = &vcpu->arch.emulate_ctxt.decode;
+		if ((emulation_type & EMULTYPE_TRAP_UD) &&
+		    (!(c->twobyte && c->b == 0x01 &&
+		      (c->modrm_reg == 0 || c->modrm_reg == 3) &&
+		       c->modrm_mod == 3 && c->modrm_rm == 1)))
+			return EMULATE_FAIL;
+
 		++vcpu->stat.insn_emulation;
 		if (r)  {
 			++vcpu->stat.insn_emulation_fail;
@@ -2639,7 +2650,8 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 		vcpu->mmio_read_completed = 1;
 		vcpu->mmio_needed = 0;
 		r = emulate_instruction(vcpu, kvm_run,
-					vcpu->arch.mmio_fault_cr2, 0, 1);
+					vcpu->arch.mmio_fault_cr2, 0,
+					EMULTYPE_NO_DECODE);
 		if (r == EMULATE_DO_MMIO) {
 			/*
 			 * Read-modify-write.  Back to userspace.
diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 20597bc..4702b04 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -415,8 +415,10 @@ enum emulation_result {
 	EMULATE_FAIL,         /* can't emulate this instruction */
 };
 
+#define EMULTYPE_NO_DECODE	    (1 << 0)
+#define EMULTYPE_TRAP_UD	    (1 << 1)
 int emulate_instruction(struct kvm_vcpu *vcpu, struct kvm_run *run,
-			unsigned long cr2, u16 error_code, int no_decode);
+			unsigned long cr2, u16 error_code, int emulation_type);
 void kvm_report_emulation_failure(struct kvm_vcpu *cvpu, const char *context);
 void realmode_lgdt(struct kvm_vcpu *vcpu, u16 size, unsigned long address);
 void realmode_lidt(struct kvm_vcpu *vcpu, u16 size, unsigned long address);
-- 
1.5.3.4


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