ima: Support platform keyring for kernel appraisal

On secure boot enabled systems, the bootloader verifies the kernel
image and possibly the initramfs signatures based on a set of keys. A
soft reboot(kexec) of the system, with the same kernel image and
initramfs, requires access to the original keys to verify the
signatures.

This patch allows IMA-appraisal access to those original keys, now
loaded on the platform keyring, needed for verifying the kernel image
and initramfs signatures.

[zohar@linux.ibm.com: only use platform keyring if it's enabled (Thiago)]
Signed-off-by: NNayna Jain <nayna@linux.ibm.com>
Reviewed-by: NMimi Zohar <zohar@linux.ibm.com>
Acked-by: NSerge Hallyn <serge@hallyn.com>
Reviewed-by: NJames Morris <james.morris@microsoft.com>
Reviewed-by: NThiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_appraise.c

@@ -289,12 +289,22 @@ int ima_appraise_measurement(enum ima_hooks func,
 	case EVM_IMA_XATTR_DIGSIG:
 		set_bit(IMA_DIGSIG, &iint->atomic_flags);
 		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
-					     (const char *)xattr_value, rc,
+					     (const char *)xattr_value,
+					     xattr_len,
 					     iint->ima_hash->digest,
 					     iint->ima_hash->length);
 		if (rc == -EOPNOTSUPP) {
 			status = INTEGRITY_UNKNOWN;
-		} else if (rc) {
+			break;
+		}
+		if (IS_ENABLED(CONFIG_INTEGRITY_PLATFORM_KEYRING) && rc &&
+		    func == KEXEC_KERNEL_CHECK)
+			rc = integrity_digsig_verify(INTEGRITY_KEYRING_PLATFORM,
+						     (const char *)xattr_value,
+						     xattr_len,
+						     iint->ima_hash->digest,
+						     iint->ima_hash->length);
+		if (rc) {
 			cause = "invalid-signature";
 			status = INTEGRITY_FAIL;
 		} else {