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

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Date:	Thu, 3 Oct 2013 08:48:03 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Alexander Graf <agraf@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.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 Thu, Oct 03, 2013 at 08:45:42AM +1000, Paul Mackerras wrote:
> On Wed, Oct 02, 2013 at 04:36:05PM +0200, Alexander Graf wrote:
> > 
> > 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?
> 
> Even if you drain all entropy out of it, wait 64 microseconds and it
> will be full again. :)  Basically it produces 64 bits every
> microsecond and puts that in a 64 entry x 64-bit FIFO buffer, which is
> what is read by the MMIO.  So there is no danger of stalling other
> processes for any significant amount of time.
> 
Even if user crates 100s guests each one of which reads hwrng in a loop?

--
			Gleb.
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