linux-kernel - Re: GET_RNG_SEED hypercall ABI? (Re: [PATCH v5 0/5] random,x86,kvm: Rework arch RNG seeds and get some from kvm)

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Message-ID: <CALCETrVW3ZQ7C6F4tab+UrUy=bX4UepPPrhVNrDtLx=ckz2EpA@mail.gmail.com>
Date:	Thu, 28 Aug 2014 17:13:18 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	"Theodore Ts'o" <tytso@....edu>, Kees Cook <keescook@...omium.org>,
	kvm list <kvm@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Bandan Das <bsd@...hat.com>, Andrew Honig <ahonig@...gle.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alok Kataria <akataria@...are.com>
Subject: Re: GET_RNG_SEED hypercall ABI? (Re: [PATCH v5 0/5] random,x86,kvm:
 Rework arch RNG seeds and get some from kvm)

On Thu, Aug 28, 2014 at 12:46 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> Il 28/08/2014 18:22, Andy Lutomirski ha scritto:
>> Is there a non-cpuid interface between QEMU and KVM for this?
>
> No.

Hmm.  Then, assuming that someone manages to allocate a
cross-hypervisor MSR number for this, what am I supposed to do in the
KVM code?  Just make it available unconditionally?  I don't see why
that wouldn't work reliably, but it seems like an odd design.

>
>> AFAICT, even turning off cpuid bits for things like async pf doesn't
>> actually disable the MSRs (which is arguably an attack surface issue).
>
> No, it doesn't.  You cannot disable instructions even if you hide CPUID
> bits, so KVM just extends this to MSRs (both native and paravirtual). It
> sometimes helps too, for example with a particular guest OS that does
> not necessary check CPUID for bits that are always present on Apple
> hardware...

But I bet that no one assumes that KVM paravirt MSRs are available
even if the feature bit isn't set.

Also, the one and only native feature flag I tested (rdtscp) actually
does work: RDTSCP seems to send #UD if QEMU is passed -cpu
host,-rdtscp.

--Andy
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