On Tue, 2019-10-01 at 12:07 -0400, Nayna wrote:

On 09/30/2019 09:04 PM, Thiago Jung Bauermann wrote:

quoted

Hello,

Hi,

quoted

quoted

diff --git a/arch/powerpc/kernel/ima_arch.c b/arch/powerpc/kernel/ima_arch.c
new file mode 100644
index 000000000000..39401b67f19e
--- /dev/null
+++ b/arch/powerpc/kernel/ima_arch.c

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2019 IBM Corporation
+ * Author: Nayna Jain
+ */
+
+#include <linux/ima.h>
+#include <asm/secure_boot.h>
+
+bool arch_ima_get_secureboot(void)
+{
+	return is_powerpc_os_secureboot_enabled();
+}
+
+/* Defines IMA appraise rules for secureboot */
+static const char *const arch_rules[] = {
+	"appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig|modsig",
+#if !IS_ENABLED(CONFIG_MODULE_SIG)
+	"appraise func=MODULE_CHECK appraise_type=imasig|modsig",
+#endif
+	NULL
+};
+
+/*
+ * Returns the relevant IMA arch policies based on the system secureboot state.
+ */
+const char *const *arch_get_ima_policy(void)
+{
+	if (is_powerpc_os_secureboot_enabled())
+		return arch_rules;
+
+	return NULL;
+}

If CONFIG_MODULE_SIG is enabled but module signatures aren't enforced,
then IMA won't enforce module signature either. x86's
arch_get_ima_policy() calls set_module_sig_enforced(). Doesn't the
powerpc version need to do that as well?

On the flip side, if module signatures are enforced by the module
subsystem then IMA will verify the signature a second time since there's
no sharing of signature verification results between the module
subsystem and IMA (this was observed by Mimi).

IMHO this is a minor issue, since module loading isn't a hot path and
the duplicate work shouldn't impact anything. But it could be avoided by
having a NULL entry in arch_rules, which arch_get_ima_policy() would
dynamically update with the "appraise func=MODULE_CHECK" rule if
is_module_sig_enforced() is true.

Thanks Thiago for reviewing.  I am wondering that this will give two 
meanings for NULL. Can we do something like below, there are possibly 
two options ?

1. Set IMA_APPRAISED in the iint->flags if is_module_sig_enforced().

OR

2. Let ima_get_action() check for is_module_sig_enforced() when policy 
is appraise and func is MODULE_CHECK.

I'm a bit hesitant about mixing the module subsystem signature
verification method with the IMA measure "template=ima-modsig" rules.
 Does it actually work?

We can at least limit verifying the same appended signature twice to
when "module.sig_enforce" is specified on the boot command line, by
changing "!IS_ENABLED(CONFIG_MODULE_SIG)" to test
"CONFIG_MODULE_SIG_FORCE".

Mimi