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

Alexander Graf
2013-10-02T14:36:14Z

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