linux-kernel - Re: [PATCH v2 1/3] KVM: x86: Deflect unknown MSR accesses to user space

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Message-ID: <8d0bae24-c83d-df89-01cb-e52cddd8ba92@amazon.com>
Date:   Fri, 31 Jul 2020 13:42:22 +0200
From:   Alexander Graf <graf@...zon.com>
To:     Jim Mattson <jmattson@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        KarimAllah Raslan <karahmed@...zon.de>,
        kvm list <kvm@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Aaron Lewis <aaronlewis@...gle.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: Deflect unknown MSR accesses to user space



On 31.07.20 05:20, Jim Mattson wrote:
> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> 
> 
> 
> On Thu, Jul 30, 2020 at 4:53 PM Jim Mattson <jmattson@...gle.com> wrote:
>>
>> On Thu, Jul 30, 2020 at 4:08 PM Alexander Graf <graf@...zon.com> wrote:
>>> Do you have a particular situation in mind where that would not be the
>>> case and where we would still want to actually complete an MSR operation
>>> after the environment changed?
>>
>> As far as userspace is concerned, if it has replied with error=0, the
>> instruction has completed and retired. If the kernel executes a
>> different instruction at CS:RIP, the state is certainly inconsistent
>> for WRMSR exits. It would also be inconsistent for RDMSR exits if the
>> RDMSR emulation on the userspace side had any side-effects.
> 
> Actually, I think there's a potential problem with interrupt delivery
> even if the instruction bytes are the same. On the second pass, an
> interrupt could be delivered on the CS:IP of a WRMSR, even though
> userspace has already emulated the WRMSR instruction. This could be
> particularly awkward if the WRMSR was to the x2APIC TPR register, and
> in fact lowered the TPR sufficiently to allow a pending interrupt to
> be delivered.

Ok, you got me convinced here :). The following flow breaks with my model:

   * rdmsr on 0x123, traps to user space
   * user space handles it, writes data into run->msr
   * kernel injects pending IRQ
   * IRQ handler does rdmsr on 0x124, which would be handled by user space
   * kernel returns value for 0x123 to the read

So yes, I agree, we have to finish the instruction handling before we 
can go back into normal operation flow.


Alex



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