[PATCH v4 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

Wanpeng Li
2018-03-30T09:06:44Z

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>
Reviewed-By: Liran Alon <liran.alon@oracle.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 | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1eb495e..c619564 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 {
@@ -4844,6 +4847,18 @@ int handle_ud(struct kvm_vcpu *vcpu)
{
enum emulation_result er;

+ 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) {
+ kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
+ return emulate_instruction(vcpu, 0) == EMULATE_DONE;
+ }
+ }
+
er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
if (er == EMULATE_USER_EXIT)
return 0;
--
2.7.4