From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:34308) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1gmKTL-00061s-4Y for qemu-devel@nongnu.org; Wed, 23 Jan 2019 10:29:48 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1gmKTH-0004jj-7D for qemu-devel@nongnu.org; Wed, 23 Jan 2019 10:29:46 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43504) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1gmKTG-0004hB-Hc for qemu-devel@nongnu.org; Wed, 23 Jan 2019 10:29:43 -0500 Date: Wed, 23 Jan 2019 16:29:25 +0100 From: Erik Skultety Message-ID: <20190123152925.GA8273@ridgehead> References: <20190118093935.GA1142@beluga.usersys.redhat.com> <65f933a2-f63c-962f-c503-43c7e84ab5e8@amd.com> <20190123125506.GA2376@beluga.usersys.redhat.com> <20190123131042.GF27270@redhat.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <25dd3d83-dbf9-5b8d-59d4-79501fe03f3c@amd.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: "Singh, Brijesh" Cc: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= , "libvir-list@redhat.com" , "qemu-devel@nongnu.org" , "dinechin@redhat.com" , "mkletzan@redhat.com" On Wed, Jan 23, 2019 at 03:02:28PM +0000, Singh, Brijesh wrote: > > > On 1/23/19 7:36 AM, Daniel P. Berrang=C3=A9 wrote: > > On Wed, Jan 23, 2019 at 02:33:01PM +0100, Erik Skultety wrote: > >> On Wed, Jan 23, 2019 at 01:24:13PM +0000, Daniel P. Berrang=C3=A9 wr= ote: > >>> On Wed, Jan 23, 2019 at 02:22:12PM +0100, Erik Skultety wrote: > >>>> On Wed, Jan 23, 2019 at 01:10:42PM +0000, Daniel P. Berrang=C3=A9 = wrote: > >>>>> 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 C= C'd on this email > >>>>>>>> about finding a solution to [1] - basically, the default permi= ssions on > >>>>>>>> /dev/sev (below) make it impossible to query for SEV platform = capabilities, > >>>>>>>> since by default we run QEMU as qemu:qemu when probing for cap= abilities. It's > >>>>>>>> worth noting is that this is only relevant to probing, since f= or a proper QEMU > >>>>>>>> VM we create a mount namespace for the process and chown all t= he nodes (needs a > >>>>>>>> SEV fix though). > >>>>>>>> > >>>>>>>> # ll /dev/sev > >>>>>>>> crw-------. 1 root root > >>>>>>>> > >>>>>>>> I suggested either force running QEMU as root for probing (des= pite the obvious > >>>>>>>> security implications) or using namespaces for probing too. Da= n argued that > >>>>>>>> this would have a significant perf impact and suggested we ask= systemd to add a > >>>>>>>> global udev rule. > >>>>>>>> > >>>>>>>> I proceeded with cloning [1] to systemd and creating an udev r= ule that I planned > >>>>>>>> on submitting to systemd upstream - the initial idea was to mi= mic /dev/kvm and > >>>>>>>> make it world accessible to which Brijesh from AMD expressed a= concern that > >>>>>>>> regular users might deplete the resources (limit on the number= of guests > >>>>>>>> allowed by the platform). > >>>>>>> > >>>>>>> > >>>>>>> During private discussion I didn't realized that we are discuss= ing a > >>>>>>> probe issue hence things I have said earlier may not be applica= ble > >>>>>>> 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= services > >>>>>>> are available through KVM driver only. Whereas the platform ser= vices 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 shoul= d not be > >>>>>>> worried about depleting the SEV ASID resources. > >>>>>>> > >>>>>>> IIRC, libvirt uses QEMP query-sev-capabilities to probe the SEV= support. > >>>>>>> QEMU executes the below sequence to complete the request: > >>>>>>> > >>>>>>> 1. Exports the platform certificates=C2=A0 (this is when /dev/s= ev is accessed). > >>>>>>> > >>>>>>> 2. Read the host MSR to determine the C-bit and reduced phys-bi= t position > >>>>>>> > >>>>>>> I don't see any reason why we can't give world a 'read' permiss= ion to > >>>>>>> /dev/sev. Anyone should be able to export the certificates and = query > >>>>>> > >>>>>> Okay, makes sense to me. The problem I see is the sev_platform_i= octl function > >>>>>> in QEMU which makes an _IOWR request, therefore the file descrip= tor being > >>>>>> 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 for IOWR > >>>>>> here - but my honest guess would be that it's because the comman= ds like > >>>>>> SEV_PDH_CERT_EXPORT or SEV_PLATFORM_STATUS need to be copied fro= m userspace to > >>>>>> kernel to instruct kernel which services we want, ergo _IOWR, is= that right? > >>>>> > >>>>> 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 anything, > >>>>> enforces which ioctls you can run when the device is only O_RDONL= Y vs O_RDWR ? > >>>> > >>>> I don't know, that's why I'm asking, because the manual didn't mak= e it any > >>>> clear for me whether there's a connection between the device permi= ssions and > >>>> ioctls that you're allowed to run. > >>>> > >>>>> > >>>>>> In any case, a fix of some sort needs to land in QEMU first, bec= ause no udev > >>>>>> rule would fix the current situation. Afterwards, I expect that = having a rule > >>>>>> like this: > >>>>>> > >>>>>> KERNEL=3D=3D"sev", GROUP=3D"kvm", MODE=3D"0644" > >>>>>> > >>>>>> and a selinux policy rule adding the kvm_device_t label, we shou= ld be fine, 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 woul= d make > >>>>> libvirt work for all currently released SEV support in kernel/qem= u. > >>>> > >>>> Sure we can, but that would make libvirt the only legitimate user = of /dev/sev > >>>> and everything else would require the admin to change the permissi= ons > >>>> explicitly so that other apps could at least retrieve the platform= info, if > >>>> it was intended to be for public use? > >>>> Additionally, we'll still get shot down by SELinux because svirt_t= wouldn't be > >>>> able to access /dev/sev by default. > >>> > >>> That's separate form probing and just needs SELinux policy to defin= e > >>> a new sev_device_t type and grant svirt_t access to it. > >> > >> I know, I misread "we can solve this entirely in libvirt" then, I th= ought you > >> the SELinux part was included in the statement, my bad then. Still, = back to the > >> original issue, we could technically do both, libvirt would have run= qemu with > >> CAP_DAC_OVERRIDE and we keep working with everything's been released= for > >> SEV in kernel/qemu and for everyone else, systemd might add 0644 for= /dev/sev, > >> that way, everyone's happy, not that I'd be a fan of libvirt often h= aving > >> to work around something because projects underneath wouldn't backpo= rt fixes to > >> all the distros we support, thus leaving the dirty work to us. > > > > Setting 0644 for /dev/sev looks unsafe to me unless someone can show > > where the permissions checks take place for the many ioctls that > > /dev/sev allows, such that only SEV_PDH_CERT_EXPORT or SEV_PLATFORM_S= TATUS > > is allowed when /dev/sev is opened by a user who doesn't have write > > permissions. > > > > Agree its not safe to do 0644. > > Currently, anyone who has access to /dev/sev (read or write) will be > able to execute SEV platform command. In other words there is no > permission check per command basis. I must admit that while developing > the driver I was under assumption that only root will ever access the > /dev/sev device hence overlooked it. But now knowing that others may > also need to access the /dev/sev, I can submit patch in kernel to do > per command access control. > > Until then, can we follow Daniel's recommendation to elevate privilege > of the probing code? If that is the case, then I absolutely agree with Dan, I'll start working= on the patches for libvirt, I'm glad I learned that there's no strict relati= on between ioctl ops and filesystem permissions unless explicitly defined. Thanks, Erik