lockdown: Enforce module signatures if the kernel is locked down

If the kernel is locked down, require that all modules have valid
signatures that we can verify.

I have adjusted the errors generated:

(1) If there's no signature (ENODATA) or we can't check it (ENOPKG,
ENOKEY), then:

(a) If signatures are enforced then EKEYREJECTED is returned.

(b) If there's no signature or we can't check it, but the kernel is
locked down then EPERM is returned (this is then consistent with
other lockdown cases).

(2) If the signature is unparseable (EBADMSG, EINVAL), the signature fails
the check (EKEYREJECTED) or a system error occurs (eg. ENOMEM), we
return the error we got.

Note that the X.509 code doesn't check for key expiry as the RTC might not
be valid or might not have been transferred to the kernel's clock yet.

[Modified by Matthew Garrett to remove the IMA integration. This will
be replaced with integration with the IMA architecture policy
patchset.]

Signed-off-by: David Howells <[e-mail]>
Signed-off-by: Matthew Garrett <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Cc: Jessica Yu <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

include/linux/security.h

···
  */
 enum lockdown_reason {
 	LOCKDOWN_NONE,
+	LOCKDOWN_MODULE_SIGNATURE,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };

init/Kconfig

···
 	  kernel build dependency so that the signing tool can use its crypto
 	  library.
 
+	  You should enable this option if you wish to use either
+	  CONFIG_SECURITY_LOCKDOWN_LSM or lockdown functionality imposed via
+	  another LSM - otherwise unsigned modules will be loadable regardless
+	  of the lockdown policy.
+
 	  !!!WARNING!!!  If you enable this option, you MUST make sure that the
 	  module DOES NOT get stripped after being signed.  This includes the
 	  debuginfo strip done by some packagers (such as rpmbuild) and

kernel/module.c

···
 #ifdef CONFIG_MODULE_SIG
 static int module_sig_check(struct load_info *info, int flags)
 {
-	int err = -ENOKEY;
+	int err = -ENODATA;
 	const unsigned long markerlen = sizeof(MODULE_SIG_STRING) - 1;
+	const char *reason;
 	const void *mod = info->hdr;
 
 	/*
···
 		err = mod_verify_sig(mod, info);
 	}
 
-	if (!err) {
+	switch (err) {
+	case 0:
 		info->sig_ok = true;
 		return 0;
+
+		/* We don't permit modules to be loaded into trusted kernels
+		 * without a valid signature on them, but if we're not
+		 * enforcing, certain errors are non-fatal.
+		 */
+	case -ENODATA:
+		reason = "Loading of unsigned module";
+		goto decide;
+	case -ENOPKG:
+		reason = "Loading of module with unsupported crypto";
+		goto decide;
+	case -ENOKEY:
+		reason = "Loading of module with unavailable key";
+	decide:
+		if (is_module_sig_enforced()) {
+			pr_notice("%s is rejected\n", reason);
+			return -EKEYREJECTED;
+		}
+
+		return security_locked_down(LOCKDOWN_MODULE_SIGNATURE);
+
+		/* All other errors are fatal, including nomem, unparseable
+		 * signatures and signature check failures - even if signatures
+		 * aren't required.
+		 */
+	default:
+		return err;
 	}
-
-	/* Not having a signature is only an error if we're strict. */
-	if (err == -ENOKEY && !is_module_sig_enforced())
-		err = 0;
-
-	return err;
 }
 #else /* !CONFIG_MODULE_SIG */
 static int module_sig_check(struct load_info *info, int flags)

security/lockdown/Kconfig

···
 config SECURITY_LOCKDOWN_LSM
 	bool "Basic module for enforcing kernel lockdown"
 	depends on SECURITY
+	select MODULE_SIG if MODULES
 	help
 	  Build support for an LSM that enforces a coarse kernel lockdown
 	  behaviour.

security/lockdown/lockdown.c

···
 
 static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_NONE] = "none",
+	[LOCKDOWN_MODULE_SIGNATURE] = "unsigned module loading",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };