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Message-ID: <CANRm+CzM0ucS1=bGxTTQ3eEfBojD5mSza0NCaiDFN_GPiuig3w@mail.gmail.com> Date: Sun, 1 Apr 2018 19:12:29 +0800 From: Wanpeng Li <kernellwp@...il.com> To: Radim Krčmář <rkrcmar@...hat.com> Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>, Andrew Cooper <andrew.cooper3@...rix.com>, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, Liran Alon <liran.alon@...cle.com> Subject: Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction" 2018-03-30 5:29 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>: > 2018-03-27 17:48-0700, Wanpeng Li: >> From: Wanpeng Li <wanpengli@...cent.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@...rix.com> >> Cc: Paolo Bonzini <pbonzini@...hat.com> >> Cc: Radim Krčmář <rkrcmar@...hat.com> >> Cc: Andrew Cooper <andrew.cooper3@...rix.com> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> >> Cc: Liran Alon <liran.alon@...cle.com> >> Signed-off-by: Wanpeng Li <wanpengli@...cent.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
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