On Sun, 2018-08-05 at 09:25 +0200, Ard Biesheuvel wrote: > Hello Chun,yi, > > On 5 August 2018 at 05:21, Lee, Chun-Yi <joeyli.kernel@xxxxxxxxx> > wrote: > > When secure boot is enabled, only signed EFI binary can access > > EFI boot service variable before ExitBootService. Which means that > > the EFI boot service variable is secure. > > > > No it, isn't, and this is a very dangerous assumption to make. > > 'Secure' means different things to different people. 'Secure boot' is > a misnomer, since it is too vague: it should be called 'authenticated > boot', and the catch is that authentication using public-key crypto > does not involve secrets at all. Hang on, let's not throw the baby out with the bathwater here. The design of "secure boot" is to create a boot time environment where only trusted code may execute. We rely on this trust guarantee when we pivot from the EFI to the MoK root of trust in shim. The reason we in Linux trust this guarantee is that it pertains to the boot environment only, so any violation would allow Windows boot to be compromised as well and we trust Microsoft's Business interests in securing windows far enough to think this would be dealt with very severely and it's an outcome the ODMs (who also add secure boot keys) are worried enough about to be very careful. The rub (and this is where I'm agreeing with Ard) is that any use case we come up with where a violation wouldn't cause a problem in windows is a use case where we cannot rely on the guarantee because Microsoft no longer has a strong business interest in policing it. This, for instance, is why we don't populate the Linux trusted keyrings with the secure boot keys (we may trust them in the boot environment where compromise would be shared with windows but we can't trust them in the Linux OS environment where it wouldn't). So this means we have to be very careful coming up with uses for secure boot that aren't strictly rooted in the guarantee as enforced by the business interests of Microsoft and the ODMs. > The UEFI variable store was not designed with confidentiality in > mind, and assuming [given the reputation of EFI on the implementation > side] that you can use it to keep secrets is rather unwise imho. Agree completely here: Microsoft doesn't use UEFI variables for confidentiality, so we shouldn't either. If you want confidentiality, use a TPM (like Microsoft does for the bitlocker key). James -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
