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Date:   Tue, 27 Mar 2018 23:55:32 -0700 (PDT)
From:   Liran Alon <liran.alon@oracle.com>
To:     <kernellwp@gmail.com>
Cc:     <konrad.wilk@oracle.com>, <rkrcmar@redhat.com>,
        <pbonzini@redhat.com>, <linux-kernel@vger.kernel.org>,
        <andrew.cooper3@citrix.com>, <kvm@vger.kernel.org>
Subject: Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate
 the next instruction"
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----- kernellwp@gmail.com wrote:

> From: Wanpeng Li <wanpengli@tencent.com>
>=20
> There is no easy way to force KVM to run an instruction through the
> emulator=20
> (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=20
> be off by default.
>=20
> A simple testcase here:
>=20
> #include <stdio.h>
> #include <string.h>
>   =20
> #define HYPERVISOR_INFO 0x40000000
>   =20
> #define CPUID(idx, eax, ebx, ecx, edx) \
>     asm volatile (\
>     "ud2a; .ascii \"kvm\"; cpuid" \
>     :"=3Db" (*ebx), "=3Da" (*eax), "=3Dc" (*ecx), "=3Dd" (*edx) \
>         :"0"(idx) ); =20
>   =20
> void main() =20
> { =20
> =09unsigned int eax, ebx, ecx, edx; =20
> =09char string[13]; =20
>   =20
> =09CPUID(HYPERVISOR_INFO, &eax, &ebx, &ecx, &edx); =20
> =09*(unsigned int *)(string + 0) =3D ebx; =20
> =09*(unsigned int *)(string + 4) =3D ecx; =20
> =09*(unsigned int *)(string + 8) =3D edx; =20
>   =20
> =09string[12] =3D 0; =20
> =09if (strncmp(string, "KVMKVMKVM\0\0\0", 12) =3D=3D 0)
> =09=09printf("kvm guest\n"); =20
> =09else =20
> =09=09printf("bare hardware\n"); =20
> }
>=20
> Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Kr=C4=8Dm=C3=A1=C5=99 <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 | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>=20
> 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 =3D
> false;
>  module_param(enable_vmware_backdoor, bool, S_IRUGO);
>  EXPORT_SYMBOL_GPL(enable_vmware_backdoor);
> =20
> +static bool __read_mostly force_emulation_prefix =3D false;
> +module_param(force_emulation_prefix, bool, S_IRUGO);
> +
>  #define KVM_NR_SHARED_MSRS 16
> =20
>  struct kvm_shared_msrs_global {
> @@ -4843,8 +4846,21 @@
> EXPORT_SYMBOL_GPL(kvm_write_guest_virt_system);
>  int handle_ud(struct kvm_vcpu *vcpu)
>  {
>  =09enum emulation_result er;
> +=09int emulation_type =3D EMULTYPE_TRAP_UD;
> +
> +=09if (force_emulation_prefix) {
> +=09=09char sig[5]; /* ud2; .ascii "kvm" */
> +=09=09struct x86_exception e;
> +
> +=09=09kvm_read_guest_virt(&vcpu->arch.emulate_ctxt,
> +=09=09=09=09kvm_get_linear_rip(vcpu), sig, sizeof(sig), &e);
> +=09=09if (memcmp(sig, "\xf\xbkvm", sizeof(sig)) =3D=3D 0) {
> +=09=09=09emulation_type =3D 0;
> +=09=09=09kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> +=09=09}
> +=09}
> =20
> -=09er =3D emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> +=09er =3D emulate_instruction(vcpu, emulation_type);
>  =09if (er =3D=3D EMULATE_USER_EXIT)
>  =09=09return 0;
>  =09if (er !=3D EMULATE_DONE)
> --=20
> 2.7.4

Reviewed-by: Liran Alon <liran.alon@oracle.com>