From: Andy Lutomirski <luto@amacapital.net>
To: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: David Matlack <dmatlack@google.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Ingo Molnar <mingo@redhat.com>, Andy Lutomirski <luto@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Eric Northup <digitaleric@google.com>
Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0
Date: Tue, 15 Mar 2016 20:48:03 -0700 [thread overview]
Message-ID: <CALCETrUm-mEdXryKesqcRzoWtFmGSJyO74dxXjGL4ZBRy4ELeQ@mail.gmail.com> (raw)
In-Reply-To: <56E8D679.9020305@linux.intel.com>
On Tue, Mar 15, 2016 at 8:43 PM, Xiao Guangrong
<guangrong.xiao@linux.intel.com> wrote:
>
>
> On 03/16/2016 03:01 AM, David Matlack wrote:
>>
>> On Mon, Mar 14, 2016 at 12:46 AM, Xiao Guangrong
>> <guangrong.xiao@linux.intel.com> wrote:
>>>
>>>
>>>
>>> On 03/12/2016 04:47 AM, David Matlack wrote:
>>>
>>>> I have not been able to trigger this bug on Linux 4.3, and suspect
>>>> it is due to this commit from Linux 4.2:
>>>>
>>>> 653f52c kvm,x86: load guest FPU context more eagerly
>>>>
>>>> With this commit, as long as the host is using eagerfpu, the guest's
>>>> fpu is always loaded just before the guest's xcr0 (vcpu->fpu_active
>>>> is always 1 in the following snippet):
>>>>
>>>> 6569 if (vcpu->fpu_active)
>>>> 6570 kvm_load_guest_fpu(vcpu);
>>>> 6571 kvm_load_guest_xcr0(vcpu);
>>>>
>>>> When the guest's fpu is loaded, irq_fpu_usable() returns false.
>>>
>>>
>>>
>>> Er, i did not see that commit introduced this change.
>>>
>>>>
>>>> We've included our workaround for this bug, which applies to Linux 3.11.
>>>> It does not apply cleanly to HEAD since the fpu subsystem was refactored
>>>> in Linux 4.2. While the latest kernel does not look vulnerable, we may
>>>> want to apply a fix to the vulnerable stable kernels.
>>>
>>>
>>>
>>> Is the latest kvm safe if we use !eager fpu?
>>
>>
>> Yes I believe so. When !eagerfpu, interrupted_kernel_fpu_idle()
>> returns "!current->thread.fpu.fpregs_active && (read_cr0() &
>> X86_CR0_TS)". This should ensure the interrupt handler never does
>> XSAVE/XRSTOR with the guest's xcr0.
>
>
>
> interrupted_kernel_fpu_idle() returns true if KVM-based hypervisor (e.g.
> QEMU)
> is not using fpu. That can not stop handler using fpu.
Why is it safe to rely on interrupted_kernel_fpu_idle? That function
is for interrupts, but is there any reason that KVM can't be preempted
(or explicitly schedule) with XCR0 having some funny value?
--Andy
--
Andy Lutomirski
AMA Capital Management, LLC
next prev parent reply other threads:[~2016-03-16 3:48 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-11 20:47 [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0 David Matlack
2016-03-11 20:47 ` [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0 is loaded David Matlack
2016-03-11 21:14 ` Andy Lutomirski
2016-03-11 21:33 ` David Matlack
2016-03-14 13:17 ` Paolo Bonzini
2016-03-15 18:27 ` Andy Lutomirski
2016-03-15 19:32 ` Paolo Bonzini
2016-03-16 3:55 ` Xiao Guangrong
2016-03-16 12:01 ` Paolo Bonzini
2016-03-14 7:46 ` [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0 Xiao Guangrong
2016-03-15 19:01 ` David Matlack
2016-03-16 3:43 ` Xiao Guangrong
2016-03-16 3:48 ` Andy Lutomirski [this message]
2016-03-16 17:11 ` David Matlack
2016-03-16 17:09 ` David Matlack
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