On Wed, 2015-10-21 at 13:21 -0400, Josh Boyer wrote: > On Wed, Oct 21, 2015 at 1:02 PM, Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx> wrote: > > On Wed, 2015-10-21 at 16:13 +0100, David Howells wrote: > >> Here's a set of patches that changes how keys are determined to be trusted > >> - currently, that's a case of whether a key has KEY_FLAG_TRUSTED set upon > >> it. A keyring can then have a flag set (KEY_FLAG_TRUSTED ONLY) that > >> indicates that only keys with this flag set may be added to that keyring. > >> > >> Further, any time an X.509 certificate is instantiated without this flag > >> set, the certificate is judged against the contents of the system trusted > >> keyring to determine whether KEY_FLAG_TRUSTED should be set upon it. > >> > >> With these patches, KEY_FLAG_TRUSTED is removed. The kernel may add > >> implicitly trusted keys to a trusted-only keyring by asserting > >> KEY_ALLOC_TRUSTED when the key is created, > > > > Ok, but only the x509 certificates built into the kernel image should be > > automatically trusted and can be added to a trusted keyring, because the > > kernel itself was signed (and verified). These certificates extend the > > (UEFI) certificate chain of trust that is rooted in hardware to the OS. > > That doesn't sound accurate to me. The cert built into the kernel > image doesn't extend the UEFI certificates. In most cases, it is a > ephemeral cert that is automatically generated at kernel build time > and then discarded. It is not chained to or derived from any of the > UEFI certs stored in the db (or mok) variables. The built-in cert is > used for module loading verification. I agree that it should be > trusted, but not really for the reason you list. Perhaps you meant > the key that the PE image of the kernel is signed with? If so, the > kernel doesn't load that. Only shim (and grub2 via shim) read that > key. This is similar to the concept of the MoK DB. Keys added to the MoK aren't signed by a UEFI key, yet they extend the UEFI secure boot certificate chain of trust. Similarly, the certificates built into the kernel image don't need to be signed by a UEFI/MoK key for it to extend the certificate chain of trust. > However, that does bring up the UEFI db/mok certs and how to deal with > those. The out-of-tree patches we have add them to the system keyring > as trusted keys. We can modify the patches to use KEY_ALLOC_TRUSTED > to preserve that functionality I suppose. Certificates are use case specific. Just because a key was trusted at the UEFI layer doesn't mean it should be trusted by the kernel (eg. Microsoft key). To illustrate this point, David Howells/David Woodhouse recently posted/upstreamed patches to differentiate how keys loaded onto the system keyring may be used. (Reference needed.) Mimi -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
