From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) by ozlabs.org (Postfix) with ESMTP id 4AEEC2C00A3 for ; Thu, 3 Oct 2013 15:48:13 +1000 (EST) Date: Thu, 3 Oct 2013 08:48:03 +0300 From: Gleb Natapov To: Paul Mackerras Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems Message-ID: <20131003054803.GU17294@redhat.com> References: <524BDD73.3020106@redhat.com> <1380704789.645.57.camel@pasglop> <668E4650-BC22-4CBF-A282-E7875DF29DB6@suse.de> <3CBF5732-E7EE-4C96-8132-6D7B77270DAF@suse.de> <20131002100224.GF17294@redhat.com> <1380722275.12149.28.camel@concordia> <029A8D6C-C23C-42B2-8C26-D76B59E2C9DD@suse.de> <524C2EAE.7090209@redhat.com> <20131002224542.GA10016@iris.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20131002224542.GA10016@iris.ozlabs.ibm.com> Cc: tytso@mit.edu, kvm@vger.kernel.org, linuxppc-dev@ozlabs.org, Alexander Graf , kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org, herbert@gondor.hengli.com.au, mpm@selenic.com, Paolo Bonzini List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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.