ima: change how MODULE_SIG_FORCE is checked on modules checking policy

This patchset ensure that IMA's modules checking policy:

measure func=MODULE_CHECK uid=0

rely on the correct value of CONFIG_MODULE_SIG_FORCE, since the way it is today the code completely ignores the module.sig_enforce cmdline param, which behaves in a OR logic with the CONFIG value (CONFIG_MODULE_SIG_FORCE || module.sig_enforce). That said, everytime a module would load, in the current checking code, when the kernel was not compiled with the CONFIG set the call to init_module syscall fails with -EACCES:

# strace -f -v modprobe <any-module> | grep init_module
init_module(0x55b9bcc9bba0, 17763, "") = -1 EACCES (Permission denied)

With this patchset the result would rely on the module.sig_enforce cmdline as well. Once the CONFIG is not set, but the param is, the result would be 'success', as it should be:

# strace -f -v modprobe <any-module> | grep init_module
init_module(0x7f9602d6e010, 386646, "") = 0

The patchset was tested in two different kernels: 4.13.6 (Fedora 27) and 4.14.0-rc4 (integrity-next tree)

Bruno E. O. Meneguele (2):
module: export module signature enforcement status
ima: check signature enforcement against cmdline param instead of
CONFIG

 include/linux/module.h            |  2 ++
 kernel/module.c                   | 10 ++++++++++
 security/integrity/ima/ima_main.c |  6 +++---
 3 files changed, 15 insertions(+), 3 deletions(-)

2.13.6

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A static variable sig_enforce is used as status var to indicate the real value of CONFIG_MODULE_SIG_FORCE, once this one is set the var will hold true, but if the CONFIG is not set the status var will hold whatever value is present in the module.sig_enforce kernel cmdline param: true when =1 and false when =0 or not present.

Considering this cmdline param take place over the CONFIG value when it's not set, other places in the kernel could misbehave since they would have only the CONFIG_MODULE_SIG_FORCE value to rely on. Exporting this status var allows the kernel to rely in the effective value of module signature enforcement, being it from CONFIG value or cmdline param.

---
 include/linux/module.h |  2 ++
 kernel/module.c        | 10 ++++++++++
 2 files changed, 12 insertions(+)

index fe5aa3736707..ddfe17e3a85c 100644
@@ -806,4 +806,6 @@ static inline bool module_sig_ok(struct module *module)
 }
 #endif	/* CONFIG_MODULE_SIG */
 
+bool is_module_sig_enforced(void);
+
 #endif /* _LINUX_MODULE_H */

index de66ec825992..d1c194b057a2 100644
@@ -278,6 +278,16 @@ static bool sig_enforce = IS_ENABLED(CONFIG_MODULE_SIG_FORCE);
 module_param(sig_enforce, bool_enable_only, 0644);
 #endif /* !CONFIG_MODULE_SIG_FORCE */
 
+/*
+ * Export sig_enforce kernel cmdline parameter to allow other subsystems rely
+ * on that instead of directly to CONFIG_MODULE_SIG_FORCE config.
+ */
+bool is_module_sig_enforced(void)
+{
+	return sig_enforce;
+}
+EXPORT_SYMBOL(is_module_sig_enforced);
+
 /* Block module loading/unloading? */
 int modules_disabled = 0;
 core_param(nomodule, modules_disabled, bint, 0);

2.13.6

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When the user requests MODULE_CHECK policy and its kernel is compiled with CONFIG_MODULE_SIG_FORCE not set, all modules would not load, just those loaded in initram time. One option the user would have would be set a kernel cmdline param (module.sig_enforce) to true, but the IMA module check code doesn't rely on this value, it checks just CONFIG_MODULE_SIG_FORCE.

This patch solves this problem checking for the exported value of module.sig_enforce cmdline param intead of CONFIG_MODULE_SIG_FORCE, which holds the effective value (CONFIG || param).

---
 security/integrity/ima/ima_main.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index e4ab8ef8016e..d11a7fcc5c8b 100644
@@ -356,12 +356,12 @@ void ima_post_path_mknod(struct dentry *dentry)
  */
 int ima_read_file(struct file *file, enum kernel_read_file_id read_id)
 {
+	bool sig_enforce = is_module_sig_enforced();
+
 	if (!file && read_id == READING_MODULE) {
-#ifndef CONFIG_MODULE_SIG_FORCE
-		if ((ima_appraise & IMA_APPRAISE_MODULES) &&
+		if (!sig_enforce && (ima_appraise & IMA_APPRAISE_MODULES) &&
 		    (ima_appraise & IMA_APPRAISE_ENFORCE))
 			return -EACCES;	/* INTEGRITY_UNKNOWN */
-#endif
 		return 0;	/* We rely on module signature checking */
 	}
 	return 0;

2.13.6

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