kexec_load(2) bypasses signature verification

[Theodore Ts'o]

On Mon, Jun 15, 2015 at 09:37:05AM -0400, Josh Boyer wrote:
> The bits that actually read Secure Boot state out of the UEFI
> variables, and apply protections to the machine to avoid compromise
> under the SB threat model.  Things like disabling the old kexec...

I don't have any real interest in using Secure Boot, but I *am*
interested in using CONFIG_KEXEC_VERIFY_SIG[1].  So perhaps we need to
have something similar to what we have with signed modules in terms of
CONFIG_MODULE_SIG_FORCE and module/sig_enforce, but for
KEXEC_VERIFY_SIG.  This would mean creating a separate flag
independent of the one Linus suggested for Secure Boot, but since we
have one for signed modules, we do have precedent for this sort of
thing.

						- Ted

[1] Yes, it doesn't buy all that much, since if the system is rooted
the adversary can just replace the kernel in /boot and force a normal,
slower reboot, but the same could be said for signed modules --- the
adversary could just replace all of /boot/vmlinux-<kver> and
/lib/modules/<kver>.  But both measures make it a tad more bit
difficult, especially for the adversary to do this replacement without
being noticed (for example linode will send me e-mail if the system
reboots normally, but not with a kexec-mediated reboot), and for cloud
systems where we don't have secure boot anyway, it's about the best we
can do.