From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:49095) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gouRi-0001Zx-2g for qemu-devel@nongnu.org; Wed, 30 Jan 2019 13:18:47 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gouRb-0004FO-Tp for qemu-devel@nongnu.org; Wed, 30 Jan 2019 13:18:42 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43834) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gouRW-0004Dl-Ep for qemu-devel@nongnu.org; Wed, 30 Jan 2019 13:18:36 -0500 Date: Wed, 30 Jan 2019 18:18:22 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20190130181822.GB15904@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <20190123132212.GA20002@beluga.usersys.redhat.com> <20190123132413.GG27270@redhat.com> <20190123133301.GB20002@beluga.usersys.redhat.com> <20190123133614.GH27270@redhat.com> <25dd3d83-dbf9-5b8d-59d4-79501fe03f3c@amd.com> <20190129161542.GG5315@beluga.usersys.redhat.com> <20190129184008.GM30796@redhat.com> <20190130080630.GI5315@beluga.usersys.redhat.com> <20190130103719.GF15904@redhat.com> <20190130133954.GA30553@beluga.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190130133954.GA30553@beluga.usersys.redhat.com> 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 30, 2019 at 02:39:54PM +0100, Erik Skultety wrote: > > > > though, we need a #ifdef check for existance of PR_CAP_AMBIENT > > > > > > > > > An alternative question I've been playing ever since we exchanged the last few > > > > > emails is that can't we wait until the ioctls are compared against permissions > > > > > in kernel so that upstream libvirt (and downstream too for that matter) doesn't > > > > > have to work around it and stick with that workaround for eternity? > > > > > > > > IIUC, the SEV feature has already shipped with distros, so we'd effectively > > > > be saying that what we already shipped is unusable to libvirt. This doesn't > > > > feel like a desirable story to me. > > > > > > It was, but it never worked, it always has been broken in this way. When we > > > were merging this upstream, we had a terrible shortage of machines and we had > > > to share, so the first person to provision the machine had already taken care > > > of the permissions in order to test so that led to this issue having been > > > overlooked until now. If it ever worked as expected and then we broke it, then > > > any fix from our side would make sense but otherwise I believe we should fix > > > this bottom up. > > > > Well technically it would work if libvirt was configured to run as > > root:root, but yes, that is not a normal or recommended configuration. > > > > Personally I have a preference for userspace solutions, as those are > > pretty straightforward to roll out to people as patches in existing > > releases. Deploying kernel updates is a higher bar to cross for an > > existing release. > > So, can you compile the prctl stuff in kernel conditionally? If so, then that's > a problem because you may end up with a platform where SEV is supported within > kernel, but you don't have the ambient stuff we have to conditionally compile > in libvirt, so you end up with broken SEV support anyway, I wanted to argue > with centos 7, but the ambient set support was backported to 3.10, so the only > distro where we'd have a problem from userspace POV would be debian 8, but then > again the kernel there is so old that neither SEV is supported there. > > I understand your point, but it also sounds very agile and I don't think that > compensating with "something that is fast" for "something that is right" is the > way to go in the long term. Especially since we almost never deprecate stuff > and we can't break compatibility. Trying to work around every issue coming > from your dependencies in your project is highly unsustainable. With the launching of VMs we've got to a place where libvirt is pretty robust about being able to grant access regardless of what the host OS has done for permissions in /dev. I think its desirable that this same flexibility extends to capabilities probing, which is somethign the dac_override approach allows. IOW, even if the kernel changes /dev/sev as previously discussed, I would keep the dac_override stuff for probing capabilities forever. This makes sure we'll work even if the distro in question has strictly locked down permissions on /dev/kvm or /dev/sev, diverging from the default udev settup Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|