On Thu, Oct 03, 2013 at 08:06:30PM +1000, Paul Mackerras wrote: > On Thu, Oct 03, 2013 at 08:48:03AM +0300, Gleb Natapov wrote: > > 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? > > Well, you can't actually have more guests running than there are cores > in a system. POWER7+ has one RNG per chip and 8 cores per chip, each > of which can run 4 threads (which have to be in the same guest). > > Michael's code uses the RNG on the same chip. Worst case therefore is > 32 threads accessing the same RNG, so a given thread might have to > wait up to 32 microseconds for its data. > OK, thanks. Even if it become an issue for some reason it is always possible to rate limit it. -- Gleb. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
