From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:53032) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gmIWv-00029r-K1 for qemu-devel@nongnu.org; Wed, 23 Jan 2019 08:25:23 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gmIIx-0005BJ-6B for qemu-devel@nongnu.org; Wed, 23 Jan 2019 08:10:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:58906) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gmIIw-0005B3-TE for qemu-devel@nongnu.org; Wed, 23 Jan 2019 08:10:55 -0500 Date: Wed, 23 Jan 2019 13:10:42 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20190123131042.GF27270@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <20190118093935.GA1142@beluga.usersys.redhat.com> <65f933a2-f63c-962f-c503-43c7e84ab5e8@amd.com> <20190123125506.GA2376@beluga.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190123125506.GA2376@beluga.usersys.redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] AMD SEV's /dev/sev permissions and probing QEMU for capabilities List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Erik Skultety Cc: "Singh, Brijesh" , "libvir-list@redhat.com" , "qemu-devel@nongnu.org" , "dinechin@redhat.com" , "mkletzan@redhat.com" On Wed, Jan 23, 2019 at 01:55:06PM +0100, Erik Skultety wrote: > On Fri, Jan 18, 2019 at 12:51:50PM +0000, Singh, Brijesh wrote: > > > > On 1/18/19 3:39 AM, Erik Skultety wrote: > > > Hi, > > > this is a summary of a private discussion I've had with guys CC'd o= n this email > > > about finding a solution to [1] - basically, the default permission= s on > > > /dev/sev (below) make it impossible to query for SEV platform capab= ilities, > > > since by default we run QEMU as qemu:qemu when probing for capabili= ties. It's > > > worth noting is that this is only relevant to probing, since for a = proper QEMU > > > VM we create a mount namespace for the process and chown all the no= des (needs a > > > SEV fix though). > > > > > > # ll /dev/sev > > > crw-------. 1 root root > > > > > > I suggested either force running QEMU as root for probing (despite = the obvious > > > security implications) or using namespaces for probing too. Dan arg= ued that > > > this would have a significant perf impact and suggested we ask syst= emd to add a > > > global udev rule. > > > > > > I proceeded with cloning [1] to systemd and creating an udev rule t= hat I planned > > > on submitting to systemd upstream - the initial idea was to mimic /= dev/kvm and > > > make it world accessible to which Brijesh from AMD expressed a conc= ern that > > > regular users might deplete the resources (limit on the number of g= uests > > > allowed by the platform). > > > > > > During private discussion I didn't realized that we are discussing a > > probe issue hence things I have said earlier may not be applicable > > during the probe. The /dev/sev is managed by the CCP (aka PSP) driver= . > > The /dev/sev is used for communicating with the SEV FW running inside > > the PSP. The SEV FW offers platform and guest specific services. The > > guest specific services are used during the guest launch, these servi= ces > > are available through KVM driver only. Whereas the platform services = can > > be invoked at anytime. A typical platform specific services are: > > > > - importing certificates > > > > - exporting certificates > > > > - querying the SEV FW version etc etc > > > > In case of the probe we are not launch SEV guest hence we should not = be > > worried about depleting the SEV ASID resources. > > > > IIRC, libvirt uses QEMP query-sev-capabilities to probe the SEV suppo= rt. > > QEMU executes the below sequence to complete the request: > > > > 1. Exports the platform certificates=C2=A0 (this is when /dev/sev is = accessed). > > > > 2. Read the host MSR to determine the C-bit and reduced phys-bit posi= tion > > > > I don't see any reason why we can't give world a 'read' permission to > > /dev/sev. Anyone should be able to export the certificates and query >=20 > Okay, makes sense to me. The problem I see is the sev_platform_ioctl fu= nction > in QEMU which makes an _IOWR request, therefore the file descriptor bei= ng > opened in sev_get_capabilities is O_RDWR. Now, I only understand ioctl = from > what I've read in the man page, so I don't quite understand the need fo= r IOWR > here - but my honest guess would be that it's because the commands like > SEV_PDH_CERT_EXPORT or SEV_PLATFORM_STATUS need to be copied from users= pace to > kernel to instruct kernel which services we want, ergo _IOWR, is that r= ight? I'm not seeing any permissions checks in the sev_ioctl() function in the kernel, so IIUC, that means any permissions are entirely based on whether you can open the /dev/sev, once open you can run any ioctl. What, if any= thing, enforces which ioctls you can run when the device is only O_RDONLY vs O_R= DWR ? > In any case, a fix of some sort needs to land in QEMU first, because no= udev > rule would fix the current situation. Afterwards, I expect that having = a rule > like this: >=20 > KERNEL=3D=3D"sev", GROUP=3D"kvm", MODE=3D"0644" >=20 > and a selinux policy rule adding the kvm_device_t label, we should be f= ine, do > we agree on that? Based on what I think I see above, this looks like a bad idea. It still looks like we can solve this entirely in libvirt by just giving the libvirt capabilities probing code CAP_DAC_OVERRIDE. This would make libvirt work for all currently released SEV support in kernel/qemu. Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|