kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE

kexec_file: split KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE

This is a preparatory patch for kexec_file_load() lockdown.  A locked down
kernel needs to prevent unsigned kernel images from being loaded with
kexec_file_load().  Currently, the only way to force the signature
verification is compiling with KEXEC_VERIFY_SIG.  This prevents loading
usigned images even when the kernel is not locked down at runtime.

This patch splits KEXEC_VERIFY_SIG into KEXEC_SIG and KEXEC_SIG_FORCE.
Analogous to the MODULE_SIG and MODULE_SIG_FORCE for modules, KEXEC_SIG
turns on the signature verification but allows unsigned images to be
loaded.  KEXEC_SIG_FORCE disallows images without a valid signature.

[Modified by David Howells such that:

 (1) verify_pefile_signature() differentiates between no-signature and
     sig-didn't-match in its returned errors.

 (2) kexec fails with EKEYREJECTED and logs an appropriate message if
     signature checking is enforced and an signature is not found, uses
     unsupported crypto or has no matching key.

 (3) kexec fails with EKEYREJECTED if there is a signature for which we
     have a key, but signature doesn't match - even if in non-forcing mode.

 (4) kexec fails with EBADMSG or some other error if there is a signature
     which cannot be parsed - even if in non-forcing mode.

 (5) kexec fails with ELIBBAD if the PE file cannot be parsed to extract
     the signature - even if in non-forcing mode.

]

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jiri Bohac <jbohac@suse.cz>
cc: Matthew Garrett <mjg59@srcf.ucam.org>
cc: Chun-Yi Lee <jlee@suse.com>
cc: kexec@lists.infradead.org

Gbp-Pq: Topic features/all/lockdown
Gbp-Pq: Name 0008-kexec_file-split-KEXEC_VERIFY_SIG-into-KEXEC_SIG-and.patch