| Message ID | 1522400804-4578-3-git-send-email-wanpengli@tencent.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
2018-03-30 02:06-0700, 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> > 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); Looking at it again, we should skip the following check if the call failed (the sig is undefined in that case). With that, or even without as we're talking about a feature that has no place in any production system, Reviewed-by: Radim Krčmář <rkrcmar@redhat.com> > + 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 >
2018-04-04 3:24 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>: > 2018-03-30 02:06-0700, 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> >> 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); > > Looking at it again, we should skip the following check if the call > failed (the sig is undefined in that case). Ok, will fallback to normal emulate_ud when this fail. > > With that, or even without as we're talking about a feature that has no > place in any production system, > > Reviewed-by: Radim Krčmář <rkrcmar@redhat.com> Thanks Radim. :) Regards, Wanpeng Li
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;