linux-kernel - Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems

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Message-Id: <C4834CF8-F81C-4B82-B1A7-1751D50AADB7@suse.de>
Date:	Wed, 2 Oct 2013 16:36:05 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Michael Ellerman <michael@...erman.id.au>,
	Gleb Natapov <gleb@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, mpm@...enic.com,
	herbert@...dor.hengli.com.au, linuxppc-dev@...abs.org,
	kvm@...r.kernel.org, kvm-ppc@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems


On 02.10.2013, at 16:33, Paolo Bonzini wrote:

> Il 02/10/2013 16:08, Alexander Graf ha scritto:
>>> The hwrng is accessible by host userspace via /dev/mem.
>> 
>> A guest should live on the same permission level as a user space
>> application. If you run QEMU as UID 1000 without access to /dev/mem, why
>> should the guest suddenly be able to directly access a memory location
>> (MMIO) it couldn't access directly through a normal user space interface.
>> 
>> It's basically a layering violation.
> 
> With Michael's earlier patch in this series, the hwrng is accessible by
> host userspace via /dev/hwrng, no?

Yes, but there's not token from user space that gets passed into the kernel to check whether access is ok or not. So while QEMU may not have permission to open /dev/hwrng it could spawn a guest that opens it, drains all entropy out of it and thus stall other processes which try to fetch entropy, no?

Maybe I haven't fully grasped the interface yet though :).


Alex

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