linux-kernel - Re: [PATCH 1/2] KVM: Exception during emulation decode should propagate

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Date:	Thu, 12 Jan 2012 12:16:31 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Nadav Amit <nadav.amit@...il.com>
CC:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>,
	Takuya Yoshikawa <takuya.yoshikawa@...il.com>,
	Nadav Amit <namit@...technion.ac.il>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: Exception during emulation decode should propagate

On 01/12/2012 11:07 AM, Nadav Amit wrote:
> >> 
> >> When I cleaned up insn_fetch(), I thought that fetching the instruction
> >> which is being executed by the guest cannot cause #PF.
> >> 
> >> The possibility that a meaningless userspace might similtaneously unmap
> >> the page, noted by Avi IIRC, was ignored intentionally, so we just fail
> >> in such a case.
> >> 
> >> Did you see any real problem?
>
> Well, I run some research project for which I emulate instructions quite
> often. I do see a real problem with Linux 3.0.0. Please note AFAIK #GP
> might occur as well during instruction fetch. I don't think failing is the
> right behavior in such case - there is no real reason to fail.
>
> Please tell me whether you are OK with KVM failing in such a scenario.

So long as it's just the guest who is affected (at the same privilege
level; we don't want guest userspace to cause a host failure).

We might have issues with userspace causing such a failure, or a nested
guest.  I see we already check for that in handle_emulation_failure()
(but not userspace).

> If not - I'll send an updated patch (in which x86_decode_insn returns
> EMULATION_OK when rc == X86EMUL_PROPAGATE_FAULT).

It guess it's better to be correct in the emulator than rely on a
failure allowing for guest-internal DoS.

-- 
error compiling committee.c: too many arguments to function

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