Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

[Liran Alon]

----- kernellwp@gmail.com wrote:

> From: Wanpeng Li <wanpengli@tencent.com>
> 
> There is no easy way to force KVM to run an instruction through the
> emulator 
> (by design as that will expose the x86 emulator as a significant
> attack-surface).
> However, we do wish to expose the x86 emulator in case we are testing
> it
> (e.g. via kvm-unit-tests). Therefore, this patch adds a "force
> emulation prefix"
> that is designed to raise #UD which KVM will trap and it's #UD
> exit-handler will
> match "force emulation prefix" to run instruction after prefix by the
> x86 emulator.
> To not expose the x86 emulator by default, we add a module parameter
> that should 
> be off by default.
> 
> A simple testcase here:
> 
> #include <stdio.h>
> #include <string.h>
>    
> #define HYPERVISOR_INFO 0x40000000
>    
> #define CPUID(idx, eax, ebx, ecx, edx) \
>     asm volatile (\
>     "ud2a; .ascii \"kvm\"; cpuid" \
>     :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
>         :"0"(idx) );  
>    
> void main()  
> {  
> 	unsigned int eax, ebx, ecx, edx;  
> 	char string[13];  
>    
> 	CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);  
> 	*(unsigned int *)(string + 0) = ebx;  
> 	*(unsigned int *)(string + 4) = ecx;  
> 	*(unsigned int *)(string + 8) = edx;  
>    
> 	string[12] = 0;  
> 	if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
> 		printf("kvm guest\n");  
> 	else  
> 		printf("bare hardware\n");  
> }
> 
> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  arch/x86/kvm/x86.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e3a60ab..40e2f78 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor =
> false;
>  module_param(enable_vmware_backdoor, bool, S_IRUGO);
>  EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
>  
> +static bool __read_mostly force_emulation_prefix = false;
> +module_param(force_emulation_prefix, bool, S_IRUGO);
> +
>  #define KVM_NR_SHARED_MSRS 16
>  
>  struct kvm_shared_msrs_global {
> @@ -4843,8 +4846,21 @@
> EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
>  int handle_ud(struct kvm_vcpu *vcpu)
>  {
>  	enum emulation_result er;
> +	int emulation_type = EMULTYPE_TRAP_UD;
> +
> +	if (force_emulation_prefix) {
> +		char sig[5]; /* ud2; .ascii "kvm" */
> +		struct x86_exception e;
> +
> +		kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
> +				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
> +		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
> +			emulation_type = 0;
> +			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +		}
> +	}
>  
> -	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> +	er = emulate_instruction(vcpu, emulation_type);
>  	if (er == EMULATE_USER_EXIT)
>  		return 0;
>  	if (er != EMULATE_DONE)
> -- 
> 2.7.4

Reviewed-by: Liran Alon <liran.alon@oracle.com>

Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

[Wanpeng Li]

2018-03-30 5:29 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
> 2018-03-27 17:48-0700, Wanpeng Li:
>> From: Wanpeng Li <wanpengli@tencent.com>
>>
>> There is no easy way to force KVM to run an instruction through the emulator
>> (by design as that will expose the x86 emulator as a significant attack-surface).
>> However, we do wish to expose the x86 emulator in case we are testing it
>> (e.g. via kvm-unit-tests). Therefore, this patch adds a "force emulation prefix"
>> that is designed to raise #UD which KVM will trap and it's #UD exit-handler will
>> match "force emulation prefix" to run instruction after prefix by the x86 emulator.
>> To not expose the x86 emulator by default, we add a module parameter that should
>> be off by default.
>>
>> A simple testcase here:
>>
>> #include <stdio.h>
>> #include <string.h>
>>
>> #define HYPERVISOR_INFO 0x40000000
>>
>> #define CPUID(idx, eax, ebx, ecx, edx) \
>>     asm volatile (\
>>     "ud2a; .ascii \"kvm\"; cpuid" \
>>     :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
>>         :"0"(idx) );
>>
>> void main()
>> {
>>       unsigned int eax, ebx, ecx, edx;
>>       char string[13];
>>
>>       CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);
>>       *(unsigned int *)(string + 0) = ebx;
>>       *(unsigned int *)(string + 4) = ecx;
>>       *(unsigned int *)(string + 8) = edx;
>>
>>       string[12] = 0;
>>       if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
>>               printf("kvm guest\n");
>>       else
>>               printf("bare hardware\n");
>> }
>>
>> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> Cc: Liran Alon <liran.alon@oracle.com>
>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>> ---
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
>>  module_param(enable_vmware_backdoor, bool, S_IRUGO);
>>  EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
>>
>> +static bool __read_mostly force_emulation_prefix = false;
>> +module_param(force_emulation_prefix, bool, S_IRUGO);
>> +
>>  #define KVM_NR_SHARED_MSRS 16
>>
>>  struct kvm_shared_msrs_global {
>> @@ -4843,8 +4846,21 @@ EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
>>  int handle_ud(struct kvm_vcpu *vcpu)
>>  {
>>       enum emulation_result er;
>> +     int emulation_type = EMULTYPE_TRAP_UD;
>> +
>> +     if (force_emulation_prefix) {
>> +             char sig[5]; /* ud2; .ascii "kvm" */
>> +             struct x86_exception e;
>> +
>> +             kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
>> +                             kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
>> +             if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
>> +                     emulation_type = 0;
>> +                     kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
>> +             }
>> +     }
>>
>> -     er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
>> +     er = emulate_instruction(vcpu, emulation_type);
>>       if (er == EMULATE_USER_EXIT)
>>               return 0;
>>       if (er != EMULATE_DONE)
>
> The code afterwards is going to inject an #UD if the emulation failed.
> I think that preserving the cpu state and forwarding the emulation
> failure to userspace would be more useful.  The change would probably be
> best as:
>
>                 if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
>                         kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
>                         return emulate_instruction(vcpu, 0) == EMULATE_DONE;
>                 }
>
> Looks great otherwise, thanks.

Do it in v4. :)

Regards,
Wanpeng Li

Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

[Radim Krčmář]

2018-03-27 17:48-0700, Wanpeng Li:
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> There is no easy way to force KVM to run an instruction through the emulator 
> (by design as that will expose the x86 emulator as a significant attack-surface).
> However, we do wish to expose the x86 emulator in case we are testing it
> (e.g. via kvm-unit-tests). Therefore, this patch adds a "force emulation prefix"
> that is designed to raise #UD which KVM will trap and it's #UD exit-handler will
> match "force emulation prefix" to run instruction after prefix by the x86 emulator.
> To not expose the x86 emulator by default, we add a module parameter that should 
> be off by default.
> 
> A simple testcase here:
> 
> #include <stdio.h>
> #include <string.h>
>    
> #define HYPERVISOR_INFO 0x40000000
>    
> #define CPUID(idx, eax, ebx, ecx, edx) \
>     asm volatile (\
>     "ud2a; .ascii \"kvm\"; cpuid" \
>     :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
>         :"0"(idx) );  
>    
> void main()  
> {  
> 	unsigned int eax, ebx, ecx, edx;  
> 	char string[13];  
>    
> 	CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);  
> 	*(unsigned int *)(string + 0) = ebx;  
> 	*(unsigned int *)(string + 4) = ecx;  
> 	*(unsigned int *)(string + 8) = edx;  
>    
> 	string[12] = 0;  
> 	if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
> 		printf("kvm guest\n");  
> 	else  
> 		printf("bare hardware\n");  
> }
> 
> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Liran Alon <liran.alon@oracle.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> @@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
>  module_param(enable_vmware_backdoor, bool, S_IRUGO);
>  EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
>  
> +static bool __read_mostly force_emulation_prefix = false;
> +module_param(force_emulation_prefix, bool, S_IRUGO);
> +
>  #define KVM_NR_SHARED_MSRS 16
>  
>  struct kvm_shared_msrs_global {
> @@ -4843,8 +4846,21 @@ EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
>  int handle_ud(struct kvm_vcpu *vcpu)
>  {
>  	enum emulation_result er;
> +	int emulation_type = EMULTYPE_TRAP_UD;
> +
> +	if (force_emulation_prefix) {
> +		char sig[5]; /* ud2; .ascii "kvm" */
> +		struct x86_exception e;
> +
> +		kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
> +				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
> +		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
> +			emulation_type = 0;
> +			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +		}
> +	}
>  
> -	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> +	er = emulate_instruction(vcpu, emulation_type);
>  	if (er == EMULATE_USER_EXIT)
>  		return 0;
>  	if (er != EMULATE_DONE)

The code afterwards is going to inject an #UD if the emulation failed.
I think that preserving the cpu state and forwarding the emulation
failure to userspace would be more useful.  The change would probably be
best as:

  		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
  			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
  			return emulate_instruction(vcpu, 0) == EMULATE_DONE;
  		}

Looks great otherwise, thanks.

(We want to use this in emulate.c kvm-unit-test that currently fails
 because of a hack that doesn't work anymore.)

[PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

[Wanpeng Li]

From: Wanpeng Li <wanpengli@tencent.com>

There is no easy way to force KVM to run an instruction through the emulator 
(by design as that will expose the x86 emulator as a significant attack-surface).
However, we do wish to expose the x86 emulator in case we are testing it
(e.g. via kvm-unit-tests). Therefore, this patch adds a "force emulation prefix"
that is designed to raise #UD which KVM will trap and it's #UD exit-handler will
match "force emulation prefix" to run instruction after prefix by the x86 emulator.
To not expose the x86 emulator by default, we add a module parameter that should 
be off by default.

A simple testcase here:

#include <stdio.h>
#include <string.h>
   
#define HYPERVISOR_INFO 0x40000000
   
#define CPUID(idx, eax, ebx, ecx, edx) \
    asm volatile (\
    "ud2a; .ascii \"kvm\"; cpuid" \
    :"=b" (*ebx), "=a" (*eax), "=c" (*ecx), "=d" (*edx) \
        :"0"(idx) );  
   
void main()  
{  
	unsigned int eax, ebx, ecx, edx;  
	char string[13];  
   
	CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx);  
	*(unsigned int *)(string + 0) = ebx;  
	*(unsigned int *)(string + 4) = ecx;  
	*(unsigned int *)(string + 8) = edx;  
   
	string[12] = 0;  
	if (strncmp(string, "KVMKVMKVM\0\0\0", 12) == 0)
		printf("kvm guest\n");  
	else  
		printf("bare hardware\n");  
}

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/x86.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e3a60ab..40e2f78 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -146,6 +146,9 @@ bool __read_mostly enable_vmware_backdoor = false;
 module_param(enable_vmware_backdoor, bool, S_IRUGO);
 EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
 
+static bool __read_mostly force_emulation_prefix = false;
+module_param(force_emulation_prefix, bool, S_IRUGO);
+
 #define KVM_NR_SHARED_MSRS 16
 
 struct kvm_shared_msrs_global {
@@ -4843,8 +4846,21 @@ EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
 int handle_ud(struct kvm_vcpu *vcpu)
 {
 	enum emulation_result er;
+	int emulation_type = EMULTYPE_TRAP_UD;
+
+	if (force_emulation_prefix) {
+		char sig[5]; /* ud2; .ascii "kvm" */
+		struct x86_exception e;
+
+		kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
+				kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
+		if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) == 0) {
+			emulation_type = 0;
+			kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
+		}
+	}
 
-	er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
+	er = emulate_instruction(vcpu, emulation_type);
 	if (er == EMULATE_USER_EXIT)
 		return 0;
 	if (er != EMULATE_DONE)
-- 
2.7.4