[edk2-devel] [PATCH 1/1] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: add security warning

[Yao, Jiewen]

Please let me summarize my feedback:

0) In general, please don't just say something is "secure" or "not secure". Without a clear threat mode, it just brings confusing to other security people. 
I recommend we describe clearly on the TCB assumption, adversary, vulnerability, etc.

1) I recommend we add a new section in README and put the information there.
Once we finish the document description, we have a clear picture on what configuration we think is legal for a production (CC or non-CC use case).
Then we can add ASSERT for the illegal configuration.

2) Because the TCB in CC and Non-CC is different, I recommend we separate the description based upon different threat models.

NOTE:
In non-CC use case, virtual SMM is considered as TCB.
In CC use case, virtual SMM is not in TCB.
If you want to discuss SMM, please describe clearly if SMM in TCB or not.

3) I don't think we need introduce a new term "stateless".
If "stateless" == no persistent variable store, I recommend we just call it "no-persistent".

In general, I could think of below the types:
A) NV storage (protected and measured by TCB): Update from SetVariable is kept in next boot and protected by TCB. The verifier can know its state with attestation.
B) NV storage (protected but not measured by TCB): Update from SetVariable is kept in next boot and protected by TCB.
C) NV storage (not protected but measured by TCB): Update from SetVariable is kept in next boot (ideally) and not protected by TCB. A non-TCB may update the Var region at any time. The verifier can know its state with attestation.
D) NV storage (not protected and not measured by TCB): Update from SetVariable is kept in next boot (ideally) and not protected by TCB. A non-TCB may update the Var region at any time. And it is not detectable.
E) no NV storage: Update from SetVariable is not kept in next boot.

Do you mean stateless == no-persistent == E)? Or something else?


Thank you
Yao Jiewen

> -----Original Message-----
> From: kraxel at redhat.com <kraxel at redhat.com>
> Sent: Tuesday, December 20, 2022 3:19 PM
> To: Yao, Jiewen <jiewen.yao at intel.com>
> Cc: devel at edk2.groups.io; Justen, Jordan L <jordan.l.justen at intel.com>; Ard
> Biesheuvel <ardb+tianocore at kernel.org>; Oliver Steffen
> <osteffen at redhat.com>; Pawel Polawski <ppolawsk at redhat.com>
> Subject: Re: [edk2-devel] [PATCH 1/1]
> OvmfPkg/QemuFlashFvbServicesRuntimeDxe: add security warning
> 
>   Hi,
> 
> > > Well, the OS can write directly to flash, bypassing the firmware.  It
> > > can update secure boot efi variables without the firmware enforcing the
> > > usual restrictions (KEK signature being required for db/dbx updates
> > > etc).
> >
> > [Jiewen] I would say: It is the typical use case. But not the only use case.
> > Typically, the BIOS shall configure the flash to prevent OS directly to flash,
> with SMM in this case. I do not disagree.
> > That feature fully really on the hardware (on a real platform) or VMM (on a
> virtual platform).
> >
> > However, in confidential computing use case, the flash is controlled by the
> untrusted entity - VMM.
> > It is *impossible* to lock down the flash, even with SMM enabled. That is
> because the VMM is also emulating SMM.
> > To rely on an untrusted entity (VMM) to provide security protection (e.g.
> SMM) is meaningless.
> 
> In the confidential computing use case there is no flash in the first
> place.  At least for the cases where the processor not the VMM manages
> guest memory (i.e SEV-SNP and TDX) the qemu patches I've seen so far
> handle it that way.
> 
> That configuration goes into the (3a) bucket.  No flash (and no other
> place to store any state which survives reboot -> stateless).  Guest
> starts with a fixed varstore coming from the firmware rom.
> 
> > The best way we can do is to enable measured boot together with secure
> > boot.
> 
> measured boot can be used to attest the VM actually runs the
> configuration it is supposed to run.  That is independent from secure
> boot, even though it is possible to combine the two and in that case
> also measure secure boot configuration.
> 
> Works in both confidential and non-confidential VMs.
> 
> > In summary, Secure boot in CC cannot resist the policy override,
> > but Measured boot in CC will detect such override, and mitigate such threat.
> > That is fine, because it is meaningless to enable CC without measured boot.
> 
> If you don't want trust the VMM, then yes, measured boot is required to
> make sure the VMM actually started the VM the way it is supposed to be
> started.
> 
> For attacks against the guest OS there isn't that much of a difference
> between CC and non-CC guests.  I think in both cases the (hacked) guest
> OS can't change the secure boot configuration.  Even in case the guest
> OS manages to modify the varstore the firmware will never use the
> modification.  For non-confidential guests the firmware will initialize
> the varstore with the pristine copy from firmware rom on reset.  For CC
> guests there is no reset in the first place (reset requires the VMM
> changing guest state), you can only kill and restart the VM, which will
> likewise start with pristine varstore state.
> 
> > > I don't see how "secure boot without SMM" is easier to understand than
> > > "stateless".
> >
> > [Jiewen] "stateless" is a new term introduced in this patch. Not in UEFI spec,
> nor in EDKII.
> > If you want to use it, please define it clearly. At least, I don't understand.
> 
> "stateless" == no persistent variable store, i.e. no flash.
> 
> > >  It also is x64-specific.
> >
> > [Jiewen] I don't understand why it is x64-specific.
> 
> SMM is x86-specific, that's why I don't like the term "secure boot
> without SMM".
> 
> > > > 4) What is the purpose of "Log a warning" ?
> > > > Is that to tell people, DON'T DO IT?
> > >
> > > Yes.
> >
> > [Jiewen] Disagree. It can work together with measured boot in CC use case.
> > If you really really want to add something, you should also check if CC is
> enabled.
> 
> See above.  CC runs without flash anyway so no warning in that case.
> 
> > > In 2022-08 and older the world is relatively simple.  We have
> > > three possible build configurations
> > >
> > >   (1) SECURE_BOOT_ENABLE=FALSE SMM_REQUIRE=FALSE
> > >       Build without secure boot support.
> > >
> > >   (2) SECURE_BOOT_ENABLE=TRUE SMM_REQUIRE=TRUE
> > >       Build with secure boot support, secure.
> > >
> > >   (3) SECURE_BOOT_ENABLE=TRUE SMM_REQUIRE=FALSE
> > >       Build with secure boot support, not secure.
> > >
> > > Linux Distributions typically provide builds for (1) and (2),
> > > so (3) existing isn't much of a problem in practice because
> > > people typically don't compile edk2 by themself.
> >
> > [Jiewen] Above description is based upon assumption that SMM is trusted.
> 
> Yes.
> 
> > It is TRUE in non-CC use case.  However, the assumption is wrong in CC
> > use case, because VMM is untrusted and SMM might colluder.  The VMM
> > may fake the flash lock in SMM, and allow OS to bypass the flash
> > protection.  Following your logic, I would say: even with SMM, it is
> > still not secure.
> 
> Not sure what you are trying to tell me.  In CC mode SMM is not
> supported in the first place, so your argument doesn't make sense to me.
> 
> > > In 2022-11 the (3) case is splitted into two:
> > >
> > >   (3a) build being used with ROM (or r/o flash):
> > >        -> this is the new "stateless secure boot" mode.
> > >   (3b) build being used with writable flash:
> > >        -> insecure configuration.
> >
> > [Jiewen] If you want to use term "stateless", please define it.
> 
> "stateless" == no persistent variable store, i.e. no flash.
> 
> > Also, I think we can separate the non-CC and CC use case, because the TCB
> is totally different.
> 
> CC goes into (3a).
> 
> take care,
>   Gerd



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