security: Refactor declaration of LSM hooks

The information about the different types of LSM hooks is scattered
in two locations i.e. union security_list_options and
struct security_hook_heads. Rather than duplicating this information
even further for BPF_PROG_TYPE_LSM, define all the hooks with the
LSM_HOOK macro in lsm_hook_defs.h which is then used to generate all
the data structures required by the LSM framework.

The LSM hooks are defined as:

  LSM_HOOK(<return_type>, <default_value>, <hook_name>, args...)

with <default_value> acccessible in security.c as:

  LSM_RET_DEFAULT(<hook_name>)
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Florent Revest <revest@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: James Morris <jamorris@linux.microsoft.com>
Link: https://lore.kernel.org/bpf/20200329004356.27286-3-kpsingh@chromium.org

security/security.c

@@ -668,6 +668,25 @@ static void __init lsm_early_task(struct task_struct *task)
 		panic("%s: Early task alloc failed.\n", __func__);
 }
 
+/*
+ * The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
+ * can be accessed with:
+ *
+ *	LSM_RET_DEFAULT(<hook_name>)
+ *
+ * The macros below define static constants for the default value of each
+ * LSM hook.
+ */
+#define LSM_RET_DEFAULT(NAME) (NAME##_default)
+#define DECLARE_LSM_RET_DEFAULT_void(DEFAULT, NAME)
+#define DECLARE_LSM_RET_DEFAULT_int(DEFAULT, NAME) \
+	static const int LSM_RET_DEFAULT(NAME) = (DEFAULT);
+#define LSM_HOOK(RET, DEFAULT, NAME, ...) \
+	DECLARE_LSM_RET_DEFAULT_##RET(DEFAULT, NAME)
+
+#include <linux/lsm_hook_defs.h>
+#undef LSM_HOOK
+
 /*
  * Hook list operation macros.
  *
@@ -1338,16 +1357,16 @@ int security_inode_getsecurity(struct inode *inode, const char *name, void **buf
 	int rc;
 
 	if (unlikely(IS_PRIVATE(inode)))
-		return -EOPNOTSUPP;
+		return LSM_RET_DEFAULT(inode_getsecurity);
 	/*
 	 * Only one module will provide an attribute with a given name.
 	 */
 	hlist_for_each_entry(hp, &security_hook_heads.inode_getsecurity, list) {
 		rc = hp->hook.inode_getsecurity(inode, name, buffer, alloc);
-		if (rc != -EOPNOTSUPP)
+		if (rc != LSM_RET_DEFAULT(inode_getsecurity))
 			return rc;
 	}
-	return -EOPNOTSUPP;
+	return LSM_RET_DEFAULT(inode_getsecurity);
 }
 
 int security_inode_setsecurity(struct inode *inode, const char *name, const void *value, size_t size, int flags)
@@ -1356,17 +1375,17 @@ int security_inode_setsecurity(struct inode *inode, const char *name, const void
 	int rc;
 
 	if (unlikely(IS_PRIVATE(inode)))
-		return -EOPNOTSUPP;
+		return LSM_RET_DEFAULT(inode_setsecurity);
 	/*
 	 * Only one module will provide an attribute with a given name.
 	 */
 	hlist_for_each_entry(hp, &security_hook_heads.inode_setsecurity, list) {
 		rc = hp->hook.inode_setsecurity(inode, name, value, size,
 								flags);
-		if (rc != -EOPNOTSUPP)
+		if (rc != LSM_RET_DEFAULT(inode_setsecurity))
 			return rc;
 	}
-	return -EOPNOTSUPP;
+	return LSM_RET_DEFAULT(inode_setsecurity);
 }
 
 int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer_size)
@@ -1740,12 +1759,12 @@ int security_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			 unsigned long arg4, unsigned long arg5)
 {
 	int thisrc;
-	int rc = -ENOSYS;
+	int rc = LSM_RET_DEFAULT(task_prctl);
 	struct security_hook_list *hp;
 
 	hlist_for_each_entry(hp, &security_hook_heads.task_prctl, list) {
 		thisrc = hp->hook.task_prctl(option, arg2, arg3, arg4, arg5);
-		if (thisrc != -ENOSYS) {
+		if (thisrc != LSM_RET_DEFAULT(task_prctl)) {
 			rc = thisrc;
 			if (thisrc != 0)
 				break;
@@ -1917,7 +1936,7 @@ int security_getprocattr(struct task_struct *p, const char *lsm, char *name,
 			continue;
 		return hp->hook.getprocattr(p, name, value);
 	}
-	return -EINVAL;
+	return LSM_RET_DEFAULT(getprocattr);
 }
 
 int security_setprocattr(const char *lsm, const char *name, void *value,
@@ -1930,7 +1949,7 @@ int security_setprocattr(const char *lsm, const char *name, void *value,
 			continue;
 		return hp->hook.setprocattr(name, value, size);
 	}
-	return -EINVAL;
+	return LSM_RET_DEFAULT(setprocattr);
 }
 
 int security_netlink_send(struct sock *sk, struct sk_buff *skb)
@@ -2315,7 +2334,7 @@ int security_xfrm_state_pol_flow_match(struct xfrm_state *x,
 				       const struct flowi *fl)
 {
 	struct security_hook_list *hp;
-	int rc = 1;
+	int rc = LSM_RET_DEFAULT(xfrm_state_pol_flow_match);
 
 	/*
 	 * Since this function is expected to return 0 or 1, the judgment