2018-03-27 17:48-0700, Wanpeng Li:
> 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>
> ---
> 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.
(We want to use this in emulate.c kvm-unit-test that currently fails
because of a hack that doesn't work anymore.)
