From: Wanpeng Li <wanpengli@xxxxxxxxxxx> 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@xxxxxxxxxx> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> Cc: Liran Alon <liran.alon@xxxxxxxxxx> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx> --- 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