1 Sep
2020
1 Sep
'20
3:12 p.m.
On 31/08/2020 18:19, Ard Biesheuvel wrote:
On Mon, 31 Aug 2020 at 20:14, François Ozog <francois.ozog@linaro.org mailto:francois.ozog@linaro.org> wrote:
On Mon, 31 Aug 2020 at 19:07, Ard Biesheuvel <ardb@kernel.org <mailto:ardb@kernel.org>> wrote: On Mon, 31 Aug 2020 at 19:30, François Ozog <francois.ozog@linaro.org <mailto:francois.ozog@linaro.org>> wrote: On Mon, 31 Aug 2020 at 17:16, Ard Biesheuvel <ardb@kernel.org <mailto:ardb@kernel.org>> wrote: On Fri, 28 Aug 2020 at 19:03, Sughosh Ganu <sughosh.ganu@linaro.org <mailto:sughosh.ganu@linaro.org>> wrote: > > hello Heinrich, > > On Fri, 28 Aug 2020 at 20:24, Heinrich Schuchardt <xypron.glpk@gmx.de <mailto:xypron.glpk@gmx.de>> > wrote: > > > On 28.08.20 14:19, Grant Likely wrote: > > > > > > > > > On 28/08/2020 12:57, Sughosh Ganu wrote: > > >> hi, > > >> I am currently working on adding support for the capsule authentication > > >> in the SetImage function of the efi firmware management protocol in > > >> u-boot. This work is part of adding functionality in u-boot for firmware > > >> updates using the uefi capsule format. > > >> > > >> The capsule authentication is done using a public key stored as a pkcs7 > > >> certificate. The uefi specification does not have any mention of how > > >> this certificate needs to be stored. This is unlike the case of the > > >> certificates used for image authentication when UEFI secure boot feature > > >> is enabled, where the certificates and hash values are stored as part of > > >> the authenticated variables like KEK, db, dbx. > > > > > > I don't think it makes sense to store the capsule authentication in the > > > KEK. PK and KEK is about the chain of trust between the platform owner > > > and one of many OSes that may be run on the platform. In the case of a > > > firmware update, it is an entirely different chain of trust. i.e. we > > > don't trust 3rd party OS vendors to also provide replacement firmware > > > images. > > > > > > The capsule update public key should be kept separately. For convenience > > > you could define another variable to hold that public key, but it would > > > be worth checking with the TF-A folks. It might make sense for BL31 to > > > be the holder of that key. > > > > > > g. > > > > > >> Can we use an authenticated variable like KEK to store the certificate > > >> used for authentication of the capsule payload. Would it make sense to > > >> have this mentioned in EBBR, or even the UEFI specification. Please let > > >> me know your thoughts. Thanks. > > > > Takahiro was working with FIT images as the content of the capsules. > > U-Boot already has RSA signing for FIT images. Isn't that enough? > > > > Cf. u-boot/doc/uImage.FIT/signature.txt > > > We do have the logic for verification of the signatures, and I have used > the same code for capsule authentication, which has been introduced by > Takahiro for image authentication. My question was about storage of the > public key certificate -- whether it should be stored as a normal uefi > variable, or as an authenticated variable. > I agree with Grant here. The scopes of signed capsule update and UEFI secure boot are entirely disjoint, and so there is no reason for the sets of certificates to overlap either, especially because the scope of signed capsule update is much narrower (i.e., any OS loader from any OS vendor could potentially be installed on a given system, whereas only a single entity publishes firmware updates for it) I'd like to see people's view on who signs what, in the following use case: - Car vendor A builds a car with tier1-1 and tier1-2 boards provided by silicon1 and silicon2. - TF-A, OP-TEE, SCMI-TA, U-Boot are provided by silicon1 for board1 and silicon2 for board2 - board1 is an android auto board and has a DRM TA on board1 provided by Netflix - board2 is an AGL board provided by tier1-2 Is the following correct? PK only cert should be issued by silicon1 and silicon2. KEK in board1 shall contain silicon1, android, Netflix certs KEK in board2 shall contain silicon2, tier1-2 certs DB shall contain the signatures of relevant images on each board. Shouldn't car vendor A have a way to insert itself in the chain of trust? So you are assuming that the fact that some parts of the firmware image are provided by different parties implies that each of those parties can control when/how those pieces get updated? Do you seriously think that, e.g., Toyota will happily let Netflix issue updates for firmware pieces that are stored on the same NOR flash as the system firmware? Netflix is one example. Insurance companies may want to have recorder code that car mechanics can't disengage because of liability. It's certainly overarching based on current best practices. In your example, *only* the car vendor controls which pieces get updated. The fact that some pieces of code have signatures that controls when/where they are permitted to execute is completely orthonogonal, and conflating these things does not make the picture any clearer. In general, firmware updates need to be authenticated to the agent that has control over the NOR flash contents, and nothing else. All this effort involving multiple signing domains, and overloading 'container' signatures as 'trusted code' signatures is a move in the wrong direction imho. At least, I'd like we make clear the trust model before we rule implementation. Based on the above use case and typical practice, what shall be in the PK, KEK and DBs (and other variables if needed)?None. PK is the root of the chain of trust for OS images, which is disjoint from the chain of trust for system firmware.
Why should we use UEFI variables for this in the first place? UEFI variables are an abstraction that permit generic UEFI clients to manipulate the boot environment. No such mechanism is needed for authenticating firmware updates, and I don't see why a OS<->firmware interoperability standard should care about such internal implementation details.
I completely agree. The firmware signing chain is 100% unrelated to the OS chain of trust (PK,KEK,DB,DBX). Firmware signing chain does not need to be exposed to the OS.
g.
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