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 12:05:25 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng li <wanpeng.li@...mail.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: don't expose syscall/sysret to intel 32-bit
 guest

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.

> +	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".

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).

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
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.

Thanks,

Paolo
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