[PATCH security-next v4 16/32] LSM: Prepare for arbitrary LSM enabling

[Kees Cook]

Before now, all the LSMs that did not specify an "enable" variable in their
struct lsm_info were considered enabled by default. This prepares to make
LSM enabling more explicit. For all LSMs without an explicit "enable"
variable, a hard-coded storage location is chosen, and all LSMs without
an external "enable" state have their state explicitly set to "enabled".

This code appears more complex than it needs to be (comma-separated
list parsing and "set" function parameter) because its use will be
expanded on in the following patches to provide more explicit enabling.

Signed-off-by: Kees Cook <keescook at chromium.org>
Reviewed-by: Casey Schaufler <casey at schaufler-ca.com>
Reviewed-by: John Johansen <john.johansen at canonical.com>
---
 security/security.c | 69 ++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 65 insertions(+), 4 deletions(-)

diff --git a/security/security.c b/security/security.c
index 4e5e67b82b7b..9459b4ee4fd9 100644
--- a/security/security.c
+++ b/security/security.c
@@ -54,17 +54,46 @@ static __initdata bool debug;
 
 static bool __init is_enabled(struct lsm_info *lsm)
 {
-	if (!lsm->enabled || *lsm->enabled)
-		return true;
+	if (WARN_ON(!lsm->enabled))
+		return false;
 
-	return false;
+	return *lsm->enabled;
 }
 
 /* Mark an LSM's enabled flag, if it exists. */
-static void __init set_enabled(struct lsm_info *lsm, bool enabled)
+static int lsm_enabled_true __initdata = 1;
+static int lsm_enabled_false __initdata = 0;
+
+static void __init default_enabled(struct lsm_info *lsm, bool enabled)
 {
+	/* If storage location already set, skip this one. */
 	if (lsm->enabled)
+		return;
+
+	/*
+	 * When an LSM hasn't configured an enable variable, we can use
+	 * a hard-coded location for storing the default enabled state.
+	 */
+	if (enabled)
+		lsm->enabled = &lsm_enabled_true;
+	else
+		lsm->enabled = &lsm_enabled_false;
+}
+
+static void __init set_enabled(struct lsm_info *lsm, bool enabled)
+{
+	if (WARN_ON(!lsm->enabled))
+		return;
+
+	if (lsm->enabled == &lsm_enabled_true) {
+		if (!enabled)
+			lsm->enabled = &lsm_enabled_false;
+	} else if (lsm->enabled == &lsm_enabled_false) {
+		if (enabled)
+			lsm->enabled = &lsm_enabled_true;
+	} else {
 		*lsm->enabled = enabled;
+	}
 }
 
 /* Is an LSM allowed to be initialized? */
@@ -127,6 +156,35 @@ static void __init major_lsm_init(void)
 	}
 }
 
+static void __init parse_lsm_enable(const char *str,
+				    void (*set)(struct lsm_info *, bool),
+				    bool enabled)
+{
+	char *sep, *name, *next;
+
+	if (!str)
+		return;
+
+	sep = kstrdup(str, GFP_KERNEL);
+	next = sep;
+	while ((name = strsep(&next, ",")) != NULL) {
+		struct lsm_info *lsm;
+
+		for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
+			if (strcmp(name, "all") == 0 ||
+			    strcmp(name, lsm->name) == 0)
+				set(lsm, enabled);
+		}
+	}
+	kfree(sep);
+}
+
+static void __init prepare_lsm_enable(void)
+{
+	/* Prepare defaults. */
+	parse_lsm_enable("all", default_enabled, true);
+}
+
 /**
  * security_init - initializes the security framework
  *
@@ -143,6 +201,9 @@ int __init security_init(void)
 	     i++)
 		INIT_HLIST_HEAD(&list[i]);
 
+	/* Figure out which LSMs are enabled and disabled. */
+	prepare_lsm_enable();
+
 	/*
 	 * Load minor LSMs, with the capability module always first.
 	 */
-- 
2.17.1