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

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Date:   Mon, 13 Nov 2017 09:32:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, yfu@...hat.com,
        Eduardo Habkost <ehabkost@...hat.com>
Subject: Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn

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

Paolo

> Regards,
> Wanpeng Li
> 
>>
>> Thanks to Eduardo Habkost for helping in the debugging of this issue.
>>
>> Reported-by: Yanan Fu <yfu@...hat.com>
>> Cc: Eduardo Habkost <ehabkost@...hat.com>
>> Cc: stable@...r.kernel.org
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>>  arch/x86/kvm/x86.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 34c85aa2e2d1..6dbed9022797 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -5722,6 +5722,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>>                         if (reexecute_instruction(vcpu, cr2, write_fault_to_spt,
>>                                                 emulation_type))
>>                                 return EMULATE_DONE;
>> +                       if (ctxt->have_exception && inject_emulated_exception(vcpu))
>> +                               return EMULATE_DONE;
>>                         if (emulation_type & EMULTYPE_SKIP)
>>                                 return EMULATE_FAIL;
>>                         return handle_emulation_failure(vcpu);
>> --
>> 1.8.3.1
>>