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SubjectRe: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit mode
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On 26/03/2018 14:25, Wanpeng Li wrote:
> 2018-03-23 23:04 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>> On 23/03/2018 15:27, Wanpeng Li wrote:
>>> 2018-03-22 21:53 GMT+08:00 Andrew Cooper <andrew.cooper3@citrix.com>:
>>>> On 22/03/18 13:39, Wanpeng Li wrote:
>>>>> 2018-03-22 20:38 GMT+08:00 Paolo Bonzini <pbonzini@redhat.com>:
>>>>>> On 22/03/2018 12:04, Andrew Cooper wrote:
>>>>>>> We've got a Force Emulation Prefix (ud2a; .ascii "xen") for doing
>>>>>>> magic. Originally, this was used for PV guests to explicitly request an
>>>>>>> emulated CPUID, but I extended it to HVM guests for "emulate the next
>>>>>>> instruction", after we had some guest user => guest kernel privilege
>>>>>>> escalations because of incorrect emulation.
>>>>>> Wanpeng, why don't you add it behind a new kvm module parameter? :)
>>>>> Great point! I will have a try. Thanks Paolo and Andrew. :)
>>>>
>>>> Using the force emulation prefix requires intercepting #UD, which is in
>>>> general a BadThing(tm) for security. Therefore, we have a build time
>>>
>>> Yeah, however kvm intercepts and emulates #UD by default, should we
>>> add a new kvm module parameter to enable it and disable by default?
>>
>> No, the module parameter should only be about the force-emulation prefix.
>
> How about something like this? (Add EmulateOnUD to cpuid, the testcase
> will use it)

I think you don't need either EmulateOnUD or EMULTYPE_TRAP_UD (the
latter only when fep=1 of course). Otherwise yes.

Paolo

>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index dd88158..80da5c6 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4772,7 +4772,7 @@ static const struct opcode twobyte_table[256] = {
> X16(D(ByteOp | DstMem | SrcNone | ModRM| Mov)),
> /* 0xA0 - 0xA7 */
> I(Stack | Src2FS, em_push_sreg), I(Stack | Src2FS, em_pop_sreg),
> - II(ImplicitOps, em_cpuid, cpuid),
> + II(EmulateOnUD | ImplicitOps, em_cpuid, cpuid),
> F(DstMem | SrcReg | ModRM | BitOp | NoWrite, em_bt),
> F(DstMem | SrcReg | Src2ImmByte | ModRM, em_shld),
> F(DstMem | SrcReg | Src2CL | ModRM, em_shld), N, N,
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9bc05f5..1825b45 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -108,6 +108,9 @@ module_param_named(enable_shadow_vmcs,
> enable_shadow_vmcs, bool, S_IRUGO);
> static bool __read_mostly nested = 0;
> module_param(nested, bool, S_IRUGO);
>
> +static bool __read_mostly fep = 0;
> +module_param(fep, bool, S_IRUGO);
> +
> static u64 __read_mostly host_xss;
>
> static bool __read_mostly enable_pml = 1;
> @@ -6215,6 +6218,27 @@ static int handle_machine_check(struct kvm_vcpu *vcpu)
> return 1;
> }
>
> +static int handle_ud(struct kvm_vcpu *vcpu)
> +{
> + enum emulation_result er;
> +
> + if (fep) {
> + 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)
> + kvm_rip_write(vcpu, kvm_rip_read(vcpu) + sizeof(sig));
> + }
> + er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> + if (er == EMULATE_USER_EXIT)
> + return 0;
> + if (er != EMULATE_DONE)
> + kvm_queue_exception(vcpu, UD_VECTOR);
> + return 1;
> +}
> +
> static int handle_exception(struct kvm_vcpu *vcpu)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> @@ -6233,14 +6257,8 @@ static int handle_exception(struct kvm_vcpu *vcpu)
> if (is_nmi(intr_info))
> return 1; /* already handled by vmx_vcpu_run() */
>
> - if (is_invalid_opcode(intr_info)) {
> - er = emulate_instruction(vcpu, EMULTYPE_TRAP_UD);
> - if (er == EMULATE_USER_EXIT)
> - return 0;
> - if (er != EMULATE_DONE)
> - kvm_queue_exception(vcpu, UD_VECTOR);
> - return 1;
> - }
> + if (is_invalid_opcode(intr_info))
> + return handle_ud(vcpu);
>
> error_code = 0;
> if (intr_info & INTR_INFO_DELIVER_CODE_MASK)
>
>
> The testcase:
>
> #include <stdio.h>
> #include <string.h>
>
> #define HYPERVISOR_INFO 0x40000000
>
> #define CPUID(idx, eax, ebx, ecx, edx)\
> asm volatile (\
> "test %1,%1;jz 1f; ud2a; .ascii \"kvm\"; 1: 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");
>
> }
>
> Regards,
> Wanpeng Li
>

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