kexec: do not verify the signature without the lockdown or mandatory signature

kexec: do not verify the signature without the lockdown or mandatory signature

Signature verification is an important security feature, to protect
system from being attacked with a kernel of unknown origin.  Kexec
rebooting is a way to replace the running kernel, hence need be secured
carefully.

In the current code of handling signature verification of kexec kernel,
the logic is very twisted.  It mixes signature verification, IMA
signature appraising and kexec lockdown.

If there is no KEXEC_SIG_FORCE, kexec kernel image doesn't have one of
signature, the supported crypto, and key, we don't think this is wrong,
Unless kexec lockdown is executed.  IMA is considered as another kind of
signature appraising method.

If kexec kernel image has signature/crypto/key, it has to go through the
signature verification and pass.  Otherwise it's seen as verification
failure, and won't be loaded.

Seems kexec kernel image with an unqualified signature is even worse
than those w/o signature at all, this sounds very unreasonable.  E.g.
If people get a unsigned kernel to load, or a kernel signed with expired
key, which one is more dangerous?

So, here, let's simplify the logic to improve code readability.  If the
KEXEC_SIG_FORCE enabled or kexec lockdown enabled, signature
verification is mandated.  Otherwise, we lift the bar for any kernel
image.

Link: http://lkml.kernel.org/r/20200602045952.27487-1-lijiang@redhat.com
Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Reviewed-by: Jiri Bohac <jbohac@suse.cz>
Acked-by: Dave Young <dyoung@redhat.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Matthew Garrett <mjg59@google.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>