linux-kernel - Re: [PATCH v3 2/2] KVM: X86: Add Force Emulation Prefix for "emulate the next instruction"

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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