Hi All, I received a bug report (private, sorry) about inability to "deploy uefi virtual machine with secureboot enabled on aarch64 kvm host". Indeed the qemu driver has some checks that would prohibit using secure boot with aarch64 virt machines, e.g. https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c... However it appears qemu does not restrict booting a firmware with keys enrolled and secure boot enabled. E.g. qemu-system-aarch64 -m 4096 -cpu host -accel kvm -smp 4 -M virt -drive if=pflash,format=raw,readonly=on,file=/usr/share/qemu/aavmf-aarch64-opensuse-code.bin -drive if=pflash,format=raw,file=/vm_images/jim/images/test/test-vars-store.bin ... seems to work fine and within the guest I see db keys loaded by kernel [ 4.782777] integrity: Loading X.509 certificate: UEFI:db [ 4.789494] integrity: Loaded X.509 cert 'Build time autogenerated kernel key: 44e3470bd0c5eb190e3292dfc42db061521184ee' [ 4.789548] integrity: Loading X.509 certificate: UEFI:db [ 4.789701] integrity: Loaded X.509 cert 'openSUSE Secure Boot Signkey: 0332fa9cbf0d88bf21924b0de82a09a54d5defc8' [ 4.789710] integrity: Loading X.509 certificate: UEFI:db [ 4.789841] integrity: Loaded X.509 cert 'SUSE Linux Enterprise Secure Boot Signkey: 3fb077b6cebc6ff2522e1c148c57c777c788e3e7' Can we consider easing the secure boot restrictions in qemuValidateDomainDefBoot?

Experimenting with the behavior on x86 raised other questions: libvirt requires the firmware to support SMM to enable secure boot. But is SMM a strict requirement for secure boot? IIUC, lack of SMM makes the securely booted stack less secure since it is easier to tamper with it, but it does not prevent securely booting the components.

When selecting firmwares manually and marking the loader secure, VM creation fails unless SMM is explicitly set in <features>. E.g. the following will fail with "unsupported configuration: Secure boot requires SMM feature enabled" <os> <type arch="x86_64" machine="q35">hvm</type> <loader readonly="yes" secure="yes" type="pflash">/usr/share/qemu/ovmf-x86_64-smm-code.bin</loader> <nvram template="/usr/share/qemu/ovmf-x86_64-smm-vars.bin"/> <boot dev="hd"/> </os> even though the descriptor file for /usr/share/qemu/ovmf-x86_64-smm-code.bin advertises secure-boot and requires-smm. Is this just a case of trying to mix old style explicit firmware selection vs firmware auto-select? I.e., if selecting the firmware explicitly, the onus is on the user to also specify any related and required config?

On Wed, Jun 08, 2022 at 12:20:20PM -0400, Andrea Bolognani wrote: > On Tue, Jun 07, 2022 at 02:57:17PM -0600, Jim Fehlig wrote: >> Hi All, >> >> I received a bug report (private, sorry) about inability to "deploy uefi >> virtual machine with secureboot enabled on aarch64 kvm host". Indeed the >> qemu driver has some checks that would prohibit using secure boot with >> aarch64 virt machines, e.g. >> >> https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c... >> >> However it appears qemu does not restrict booting a firmware with keys >> enrolled and secure boot enabled. E.g. >> >> qemu-system-aarch64 -m 4096 -cpu host -accel kvm -smp 4 -M virt -drive if=pflash,format=raw,readonly=on,file=/usr/share/qemu/aavmf-aarch64-opensuse-code.bin >> -drive >> if=pflash,format=raw,file=/vm_images/jim/images/test/test-vars-store.bin ... >> >> seems to work fine and within the guest I see db keys loaded by kernel >> >> [ 4.782777] integrity: Loading X.509 certificate: UEFI:db >> [ 4.789494] integrity: Loaded X.509 cert 'Build time autogenerated kernel >> key: 44e3470bd0c5eb190e3292dfc42db061521184ee' >> [ 4.789548] integrity: Loading X.509 certificate: UEFI:db >> [ 4.789701] integrity: Loaded X.509 cert 'openSUSE Secure Boot Signkey: >> 0332fa9cbf0d88bf21924b0de82a09a54d5defc8' >> [ 4.789710] integrity: Loading X.509 certificate: UEFI:db >> [ 4.789841] integrity: Loaded X.509 cert 'SUSE Linux Enterprise Secure >> Boot Signkey: 3fb077b6cebc6ff2522e1c148c57c777c788e3e7' >> >> Can we consider easing the secure boot restrictions in qemuValidateDomainDefBoot? > > Will such a configuration refuse to boot an unsigned guest OS? Is it > reasonably tamper-proof (see below)? If the answer to both of these > question is yes, then relaxing the check sounds reasonable. > >> Experimenting with the behavior on x86 raised other questions: >> >> libvirt requires the firmware to support SMM to enable secure boot. But is >> SMM a strict requirement for secure boot? IIUC, lack of SMM makes the >> securely booted stack less secure since it is easier to tamper with it, but >> it does not prevent securely booting the components. > > I've been looking into this recently with the help of a colleague > who's way more knowledgeable about the topic than I could possibly > ever be and the short explanation is that, if you don't have SMM > enabled and the loader marked as secure, then the guest OS will be > able to modify the chain of trust stored in NVRAM, thus defeating the > purpose of enabling secure boot in the first place. > > I don't think we should relax these requirements, as doing so has the > potential to lure users into a false sense of security. Note SMM is a x86 specific concept. I agree we shouldn't remove that rule for x86, because that makes SecureBoot as robust as a chocolate teapot.

Jim started by talking about aarch64 though, and there's no SSM concept there. In fact everything in qemuValidateDomainDefBoot is entirely x86 specific right now wrt secure boot. So this method will definitely need work for aarch64.

>> When selecting firmwares manually and marking the loader secure, VM creation >> fails unless SMM is explicitly set in <features>. E.g. the following will >> fail with "unsupported configuration: Secure boot requires SMM feature >> enabled" >> >> <os> >> <type arch="x86_64" machine="q35">hvm</type> >> <loader readonly="yes" secure="yes" >> type="pflash">/usr/share/qemu/ovmf-x86_64-smm-code.bin</loader> >> <nvram template="/usr/share/qemu/ovmf-x86_64-smm-vars.bin"/> >> <boot dev="hd"/> >> </os> >> >> even though the descriptor file for /usr/share/qemu/ovmf-x86_64-smm-code.bin >> advertises secure-boot and requires-smm. Is this just a case of trying to >> mix old style explicit firmware selection vs firmware auto-select? I.e., if >> selecting the firmware explicitly, the onus is on the user to also specify >> any related and required config? > > The manual firmware selection interface is too complex to be > reasonably useful to regular users, so I think it's fair to consider > it a development tool / legacy interface at this point. Anyone who's > not a libvirt or firmware developer should really use the > feature-based firmware selection interface instead. > > I have a couple of improvements to the automatic firmware selection > feature and the corresponding documentation in mind. Hopefully I'll > get around to post patches over the next few days. To expand on this, if you're using auto-selection it should all "just work". If you are not using auto-selection, then the *only* thing that the firmware descriptors are used for is to locate a template for the variables file. Anything else is ignored, as the user is considered to have said they want todo things manually. This is why the 'requires-smm' flag has no effect in Jim's example above.

On Wed, Jun 08, 2022 at 11:46:43AM -0600, Jim Fehlig wrote: > On 6/8/22 10:28, Daniel P. Berrangé wrote: >> On Wed, Jun 08, 2022 at 12:20:20PM -0400, Andrea Bolognani wrote: >>> On Tue, Jun 07, 2022 at 02:57:17PM -0600, Jim Fehlig wrote: >>>> Hi All, >>>> >>>> I received a bug report (private, sorry) about inability to "deploy uefi >>>> virtual machine with secureboot enabled on aarch64 kvm host". Indeed the >>>> qemu driver has some checks that would prohibit using secure boot with >>>> aarch64 virt machines, e.g. >>>> >>>> https://gitlab.com/libvirt/libvirt/-/blob/master/src/qemu/qemu_validate.c... >>>> >>>> However it appears qemu does not restrict booting a firmware with keys >>>> enrolled and secure boot enabled. E.g. >>>> >>>> qemu-system-aarch64 -m 4096 -cpu host -accel kvm -smp 4 -M virt -drive if=pflash,format=raw,readonly=on,file=/usr/share/qemu/aavmf-aarch64-opensuse-code.bin >>>> -drive >>>> if=pflash,format=raw,file=/vm_images/jim/images/test/test-vars-store.bin ... >>>> >>>> seems to work fine and within the guest I see db keys loaded by kernel >>>> >>>> [ 4.782777] integrity: Loading X.509 certificate: UEFI:db >>>> [ 4.789494] integrity: Loaded X.509 cert 'Build time autogenerated kernel >>>> key: 44e3470bd0c5eb190e3292dfc42db061521184ee' >>>> [ 4.789548] integrity: Loading X.509 certificate: UEFI:db >>>> [ 4.789701] integrity: Loaded X.509 cert 'openSUSE Secure Boot Signkey: >>>> 0332fa9cbf0d88bf21924b0de82a09a54d5defc8' >>>> [ 4.789710] integrity: Loading X.509 certificate: UEFI:db >>>> [ 4.789841] integrity: Loaded X.509 cert 'SUSE Linux Enterprise Secure >>>> Boot Signkey: 3fb077b6cebc6ff2522e1c148c57c777c788e3e7' >>>> >>>> Can we consider easing the secure boot restrictions in qemuValidateDomainDefBoot? >>> >>> Will such a configuration refuse to boot an unsigned guest OS? Is it >>> reasonably tamper-proof (see below)? If the answer to both of these >>> question is yes, then relaxing the check sounds reasonable. >>> >>>> Experimenting with the behavior on x86 raised other questions: >>>> >>>> libvirt requires the firmware to support SMM to enable secure boot. But is >>>> SMM a strict requirement for secure boot? IIUC, lack of SMM makes the >>>> securely booted stack less secure since it is easier to tamper with it, but >>>> it does not prevent securely booting the components. >>> >>> I've been looking into this recently with the help of a colleague >>> who's way more knowledgeable about the topic than I could possibly >>> ever be and the short explanation is that, if you don't have SMM >>> enabled and the loader marked as secure, then the guest OS will be >>> able to modify the chain of trust stored in NVRAM, thus defeating the >>> purpose of enabling secure boot in the first place. >>> >>> I don't think we should relax these requirements, as doing so has the >>> potential to lure users into a false sense of security. >> >> Note SMM is a x86 specific concept. I agree we shouldn't remove that >> rule for x86, because that makes SecureBoot as robust as a chocolate >> teapot. > > Ok, got it :-). > >> Jim started by talking about aarch64 though, and there's no SSM concept >> there. In fact everything in qemuValidateDomainDefBoot is entirely >> x86 specific right now wrt secure boot. So this method will definitely >> need work for aarch64. > > Agreed, it's just not clear to me how at this point. Without similar SMM > protection of the chain of trust, it seems pointless to support secure boot > in libvirt. I think we need an ARM expert to explain the rules about SecureBoot on aarch64. Given SMM doesn't exist outside x86, it may be fine to just enable secureboot unconditionally on aarch64 and have it be genuinely secure. I simply don't know enough in this respect. With regards, Daniel

> I think we need an ARM expert to explain the rules about SecureBoot > on aarch64. Given SMM doesn't exist outside x86, it may be fine to > just enable secureboot unconditionally on aarch64 and have it be > genuinely secure. I simply don't know enough in this respect. Unlikely. The firmware needs some way to store state (i.e. uefi variables) somewhere where the guest OS can't tamper with it. On x86 SMM mode provides that. Flash can be configured to be writable in SMM mode only, so the guest OS can't change things directly but has to route all update requests through the firmware. The arm architecture has a concept called TrustZone which can as far I know can provide simliar guarantees. Checking upstream edk2 git log finds me this... commit 6b46d77243e02d23ce922803998e01277fe9f399 Author: Supreeth Venkatesh <supreeth.venkatesh(a)arm.com&gt; Date: Fri Jul 13 23:05:27 2018 +0800 StandaloneMmPkg/Core: Implementation of Standalone MM Core Module. Management Mode (MM) is a generic term used to describe a secure execution environment provided by the CPU and related silicon that is entered when the CPU detects a MMI. For x86 systems, this can be implemented with System Management Mode (SMM). For ARM systems, this can be implemented with TrustZone (TZ). [ ... snip ... ] ... but not much beyond that. So I don't think edk2 has support implemented, and I'm also not sure where we stand with qemu. I suspect it'll be quite a bit of work to get everything going.

Hi All, I received a bug report (private, sorry) about inability to "deploy uefi virtual machine with secureboot enabled on aarch64 kvm host". Indeed the qemu driver has some checks that would prohibit using secure boot with aarch64 virt machines, e.g.

libvirt requires the firmware to support SMM to enable secure boot. But is SMM a strict requirement for secure boot? IIUC, lack of SMM makes the securely booted stack less secure since it is easier to tamper with it, but it does not prevent securely booting the components.