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

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Date:   Mon, 13 Nov 2017 15:15:43 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.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>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: inject exceptions produced by x86_decode_insn

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.

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
>