linux-kernel - Re: [PATCH] KVM: x86: don't expose syscall/sysret to intel 32-bit guest

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Date:	Thu, 19 Nov 2015 20:01:49 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Wanpeng li <wanpeng.li@...mail.com>, kvm <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: don't expose syscall/sysret to intel 32-bit guest

2015-11-19 19:05 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> On 19/11/2015 11:45, Wanpeng li wrote:
>> Intel cpu doesn't support syscall/sysret in non 64-bit mode which
>> is different from AMD. Expose syscall/sysret to intel 32-bit guest
>> just makes no sense and leads to #UD which will confuse the users.
>>
>> This patch disable expose syscall/sysret to intel 32-bit guest.
>>
>> Signed-off-by: Wanpeng li <wanpeng.li@...mail.com>
>> ---
>>  arch/x86/kvm/cpuid.c | 16 ++++++++++++++++
>>  arch/x86/kvm/x86.c   |  1 +
>>  2 files changed, 17 insertions(+)
>>
>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>> index 6525e92..f8ddca6 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -62,6 +62,7 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>>  {
>>       struct kvm_cpuid_entry2 *best;
>>       struct kvm_lapic *apic = vcpu->arch.apic;
>> +     bool vendor_intel = false;
>>
>>       best = kvm_find_cpuid_entry(vcpu, 1, 0);
>>       if (!best)
>> @@ -114,6 +115,21 @@ int kvm_update_cpuid(struct kvm_vcpu *vcpu)
>>       vcpu->arch.maxphyaddr = cpuid_query_maxphyaddr(vcpu);
>>
>>       kvm_pmu_refresh(vcpu);
>> +
>> +     /* Don't expose syscall/sysret to intel non 64-bit mode guest */
>> +     best = kvm_find_cpuid_entry(vcpu, 0x00000000, 0x00000000);
>> +     if (best && best->ebx == 0x756e6547 && best->ecx == 0x6c65746e &&
>> +             best->edx == 0x49656e69)
>> +             vendor_intel = true;
>
> You can use X86EMUL_CPUID_VENDOR_GenuineIntel_ebx/ecx/edx instead here.

Ok.

>
>> +     best = kvm_find_cpuid_entry(vcpu, 0x80000001, 0);
>> +     if (best && vendor_intel) {
>> +             if (!is_long_mode(vcpu))
>> +                     best->edx &= ~F(SYSCALL);
>> +             else
>> +                     best->edx |= F(SYSCALL);
>
> This is not correct.  As far as I know, the SYSCALL bit is always
> present in CPUID, even if the machine is running in 32-bit mode; CPUID
> documentation (SDM Volume 2) explicitly documents bit 11 as "Bit 11:
> SYSCALL/SYSRET available in 64-bit mode".

No, I try a 32-bit linux host kernel, cpuid tool shows that SYSCALL
bit is not set.

>
> So there are two possibilities here:
>
> 1) Clear F(SYSCALL) in kvm_update_cpuid, like you are doing here but
> only if F(LM) is already clear (in addition to the vendor being Intel).

Good point. :-)

>
> 2) do nothing, declare this a userspace bug.  Linux knows that only AMD
> machines support SYSCALL in 32-bit mode (there is a separate feature bit
> X86_FEATURE_SYSCALL32 that is only set by arch/x86/kernel/cpu/amd.c).
> Are you fixing an issue with Windows?  If so, with what QEMU command

No.

> line?  But if it's not an issue with a real guest, doing nothing would
> be my favorite option...
>
> In any case, set_efer doesn't need to call kvm_update_cpuid.

Btw, I also try a 32-bit linux guest in vmware on my macbook, it also
doesn't expose
syscall/sysret.

Regards,
Wanpeng Li
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