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

Radim Krčmář
2018-04-03T19:24:08Z

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
>