LSM: Create lsm_module_list system call

Create a system call to report the list of Linux Security Modules
that are active on the system. The list is provided as an array
of LSM ID numbers.

The calling application can use this list determine what LSM
specific actions it might take. That might include chosing an
output format, determining required privilege or bypassing
security module specific behavior.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

Documentation/userspace-api/lsm.rst

@@ -57,6 +57,9 @@ Get the security attributes of the current process
 .. kernel-doc:: security/lsm_syscalls.c
     :identifiers: sys_lsm_get_self_attr
 
+.. kernel-doc:: security/lsm_syscalls.c
+    :identifiers: sys_lsm_module_list
+
 Additional documentation
 ========================
 

include/linux/syscalls.h

@@ -1061,6 +1061,7 @@ asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long l
 					    unsigned long flags);
 asmlinkage long sys_lsm_get_self_attr(struct lsm_ctx *ctx, size_t *size,
 				      int flags);
+asmlinkage long sys_lsm_module_list(u32 *ids, size_t *size, int flags);
 
 /*
  * Architecture-specific system calls

kernel/sys_ni.c

@@ -264,6 +264,7 @@ COND_SYSCALL(mremap);
 
 /* security/lsm_syscalls.c */
 COND_SYSCALL(lsm_get_self_attr);
+COND_SYSCALL(lsm_module_list);
 
 /* security/keys/keyctl.c */
 COND_SYSCALL(add_key);

security/lsm_syscalls.c

@@ -180,3 +180,44 @@ SYSCALL_DEFINE3(lsm_get_self_attr,
 	kfree(final);
 	return rc;
 }
+
+/**
+ * sys_lsm_module_list - Return a list of the active security modules
+ * @ids: the LSM module ids
+ * @size: size of @ids, updated on return
+ * @flags: reserved for future use, must be zero
+ *
+ * Returns a list of the active LSM ids. On success this function
+ * returns the number of @ids array elements. This value may be zero
+ * if there are no LSMs active. If @size is insufficient to contain
+ * the return data -E2BIG is returned and @size is set to the minimum
+ * required size. In all other cases a negative value indicating the
+ * error is returned.
+ */
+SYSCALL_DEFINE3(lsm_module_list,
+		u32 __user *, ids,
+		size_t __user *, size,
+		u64, flags)
+{
+	size_t total_size = lsm_active_cnt * sizeof(*ids);
+	size_t usize;
+	int i;
+
+	if (flags)
+		return -EINVAL;
+
+	if (get_user(usize, size))
+		return -EFAULT;
+
+	if (put_user(total_size, size) != 0)
+		return -EFAULT;
+
+	if (usize < total_size)
+		return -E2BIG;
+
+	for (i = 0; i < lsm_active_cnt; i++)
+		if (put_user(lsm_idlist[i]->id, ids++))
+			return -EFAULT;
+
+	return lsm_active_cnt;
+}