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. -Sumit > > > Also, do update documentation corresponding to CAAM as a trusted keys > > backend. > > Yes. The documentation should be updated for CAAM and it should describe > how the key material is derived. Will do so for v2. > > Cheers, > Ahmad > > > > > [1] > > https://git.kernel.org/pub/scm/linux/kernel/git/jarkko/linux-tpmdd.git/tree/Documentation/security/keys/trusted-encrypted.rst#n87 > > > > -Sumit > > > >> Cheers, > >> Ahmad > >> > >>> > >>> thanks, > >>> > >>> Mimi > >>> > >>> [1] > >>> https://lore.kernel.org/linux-integrity/bca04d5d9a3b764c9b7405bba4d4a3c035f2a...@alpmbapa12.e2k.ad.ge.com/ > >>> > >>> > >>> > >> > >> -- > >> Pengutronix e.K. | | > >> Steuerwalder Str. 21 | http://www.pengutronix.de/ | > >> 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | > >> Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | > > > > -- > Pengutronix e.K. | | > Steuerwalder Str. 21 | http://www.pengutronix.de/ | > 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | > Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
