On Tue, Mar 30, 2021 at 12:56:41PM +0530, Sumit Garg wrote: > On Mon, 29 Mar 2021 at 01:07, Jarkko Sakkinen <jar...@kernel.org> wrote: > > > > On Sat, Mar 27, 2021 at 01:41:24PM +0100, David Gstir wrote: > > > Hi! > > > > > > > On 25.03.2021, at 06:26, Sumit Garg <sumit.g...@linaro.org> wrote: > > > > > > > > On Wed, 24 Mar 2021 at 19:37, Ahmad Fatoum <a.fat...@pengutronix.de> > > > > wrote: > > > >> > > > >> Hello Sumit, > > > >> > > > >> On 24.03.21 11:47, Sumit Garg wrote: > > > >>> On Wed, 24 Mar 2021 at 14:56, Ahmad Fatoum <a.fat...@pengutronix.de> > > > >>> wrote: > > > >>>> > > > >>>> Hello Mimi, > > > >>>> > > > >>>> On 23.03.21 19:07, Mimi Zohar wrote: > > > >>>>> On Tue, 2021-03-23 at 17:35 +0100, Ahmad Fatoum wrote: > > > >>>>>> On 21.03.21 21:48, Horia Geantă wrote: > > > >>>>>>> caam has random number generation capabilities, so it's worth > > > >>>>>>> using that > > > >>>>>>> by implementing .get_random. > > > >>>>>> > > > >>>>>> If the CAAM HWRNG is already seeding the kernel RNG, why not use > > > >>>>>> the kernel's? > > > >>>>>> > > > >>>>>> Makes for less code duplication IMO. > > > >>>>> > > > >>>>> Using kernel RNG, in general, for trusted keys has been discussed > > > >>>>> before. Please refer to Dave Safford's detailed explanation for > > > >>>>> not > > > >>>>> using it [1]. > > > >>>> > > > >>>> The argument seems to boil down to: > > > >>>> > > > >>>> - TPM RNG are known to be of good quality > > > >>>> - Trusted keys always used it so far > > > >>>> > > > >>>> Both are fine by me for TPMs, but the CAAM backend is new code and > > > >>>> neither point > > > >>>> really applies. > > > >>>> > > > >>>> get_random_bytes_wait is already used for generating key material > > > >>>> elsewhere. > > > >>>> Why shouldn't new trusted key backends be able to do the same thing? > > > >>>> > > > >>> > > > >>> Please refer to documented trusted keys behaviour here [1]. New > > > >>> trusted key backends should align to this behaviour and in your case > > > >>> CAAM offers HWRNG so we should be better using that. > > > >> > > > >> Why is it better? > > > >> > > > >> Can you explain what benefit a CAAM user would have if the trusted key > > > >> randomness comes directly out of the CAAM instead of indirectly from > > > >> the kernel entropy pool that is seeded by it? > > > > > > > > IMO, user trust in case of trusted keys comes from trusted keys > > > > backend which is CAAM here. If a user doesn't trust that CAAM would > > > > act as a reliable source for RNG then CAAM shouldn't be used as a > > > > trust source in the first place. > > > > > > > > And I think building user's trust for kernel RNG implementation with > > > > multiple entropy contributions is pretty difficult when compared with > > > > CAAM HWRNG implementation. > > > > > > Generally speaking, I’d say trusting the CAAM RNG and trusting in it’s > > > other features are two separate things. However, reading through the CAAM > > > key blob spec I’ve got here, CAAM key blob keys (the keys that secure a > > > blob’s > > > content) are generated using its internal RNG. So I’d save if the CAAM RNG > > > is insecure, so are generated key blobs. Maybe somebody with more insight > > > into the CAAM internals can verify that, but I don’t see any point in > > > using > > > the kernel’s RNG as long as we let CAAM generate the key blob keys for us. > > > > Here's my long'ish analysis. Please read it to the end if by ever means > > possible, and apologies, I usually try to keep usually my comms short, but > > this requires some more meat than the usual. > > > > The Bad News > > ============ > > > > Now that we add multiple hardware trust sources for trusted keys, will > > there ever be a scenario where a trusted key is originally sealed with a > > backing hardware A, unsealed, and resealed with hardware B? > > > > The hardware and vendor neutral way to generate the key material would be > > unconditionally always just the kernel RNG. > > > > CAAM is actually worse than TCG because it's not even a standards body, if > > I got it right. Not a lot but at least a tiny fraction. > > > > This brings an open item in TEE patches: trusted_tee_get_random() is an > > issue in generating kernel material. I would rather replace that with > > kernel RNG *for now*, because the same open question applies also to ARM > > TEE. It's also a single company controlled backing technology. > > > > By all practical means, I do trust ARM TEE in my personal life but this is > > not important. > > > > CAAM *and* TEE backends break the golden rule of putting as little trust as > > possible to anything, even not anything weird is clear at sight, as > > security is essentially a game of known unknowns and unknown unknowns. > > > > Unfortunately, TPM trusted keys started this bad security practice, and > > obviously it cannot be fixed without breaking uapi backwards compatibility. > > > > This leaves me exactly two rational options: > > > > A. Add a patch to remove trusted_tee_get_random() and use kernel RNG > > instead. > > B. Drop the whole TEE patch set up until I have good reasons to believe > > that it's the best possible idea ever to use TEE RNG. > > > > Doing does (A) does not disclude of doing (B) later on, if someone some > > day sends a patch with sound reasoning. > > > > It's also good to understand that when some day a vendor D, other than TCG, > > CAAM or ARM, comes up, we need to go again this lenghty and messy > > discussion. Now this already puts an already accepted patch set into a > > risk, because by being a responsible maintainer I would have legit reasons > > just simply to drop it. > > > > OK, but.... > > > > The GOOD News > > ============= > > > > So there's actually option (C) that also fixes the TPM trustd keys issue: > > > > Add a new kernel patch, which: > > > > 1. Adds the use of kernel RNG as a boot option. > > 2. If this boot option is not active, the subsystem will print a warning > > to klog denoting this. > > 3. Default is of course vendor RNG given the bad design issue in the TPM > > trusted keys, but the warning in klog will help to address it at least > > a bit. > > 4. Document all this to Documentation/security/keys/trusted-encrypted.rst. > > > > I'd prefer the choice between A, B and C be concluded rather sooner than > > later. > > Option (C) sounds reasonable to me but I would rather prefer an info > message rather than warning as otherwise it would reflect that we are > enforcing kernel RNG choice for a user to trust upon.
I gave some though on this. I take TEE as it is but I'd expect the CAAM patch set sort out this option with some patch. /Jarkko
