linux-kernel - Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode

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Message-ID: <CANRm+CwuumWuDZXXjODSxein_2b3yJKZGo+fKBJW9p97xzNHXQ@mail.gmail.com>
Date:   Thu, 30 Nov 2017 17:20:12 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, yfu@...hat.com
Subject: Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn

2017-11-30 2:42 GMT+08:00 Eduardo Habkost <ehabkost@...hat.com>:
> On Wed, Nov 29, 2017 at 12:44:42PM +0100, Paolo Bonzini wrote:
>> On 29/11/2017 12:44, Eduardo Habkost wrote:
>> > On Mon, Nov 13, 2017 at 09:32:09AM +0100, Paolo Bonzini wrote:
>> >> On 13/11/2017 08:15, Wanpeng Li wrote:
>> >>> 2017-11-10 17:49 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>> >>>> Sometimes, a processor might execute an instruction while another
>> >>>> processor is updating the page tables for that instruction's code page,
>> >>>> but before the TLB shootdown completes.  The interesting case happens
>> >>>> if the page is in the TLB.
>> >>>>
>> >>>> In general, the processor will succeed in executing the instruction and
>> >>>> nothing bad happens.  However, what if the instruction is an MMIO access?
>> >>>> If *that* happens, KVM invokes the emulator, and the emulator gets the
>> >>>> updated page tables.  If the update side had marked the code page as non
>> >>>> present, the page table walk then will fail and so will x86_decode_insn.
>> >>>>
>> >>>> Unfortunately, even though kvm_fetch_guest_virt is correctly returning
>> >>>> X86EMUL_PROPAGATE_FAULT, x86_decode_insn's caller treats the failure as
>> >>>> a fatal error if the instruction cannot simply be reexecuted (as is the
>> >>>> case for MMIO).  And this in fact happened sometimes when rebooting
>> >>>> Windows 2012r2 guests.  Just checking ctxt->have_exception and injecting
>> >>>> the exception if true is enough to fix the case.
>> >>>
>> >>> I found the only place which can set ctxt->have_exception is in the
>> >>> function x86_emulate_insn(), and x86_decode_insn() will not set
>> >>> ctxt->have_exception even if kvm_fetch_guest_virt() returns
>> >>> X86_EMUL_PROPAGATE_FAULT.
>> >>
>> >> Hmm, you're right.  Looks like Yanan has been (un)lucky when trying out
>> >> this patch! :(
>> >>
>> >> Yanan, can you double check that you can reproduce the issue with an
>> >> unpatched kernel?  I will work on a kvm-unit-tests testcsae
>> >
>> > We don't have a kvm-unit-tests reproducer for this yet, right?
>> >
>> > I'm considering trying to write one, but I don't want to
>> > duplicate work.
>>
>> No, I haven't written one yet.
>
> The reproducer (not a full test case) is quite simple, see patch below.

I can also have a look if there is a formal test case. :)

Regards,
Wanpeng Li