tomoyo: allow building as a loadable LSM module

One of concerns for enabling TOMOYO in prebuilt kernels is that distributor
wants to avoid bloating kernel packages. Although boot-time kernel command
line options allows selecting built-in LSMs to enable, file size increase
of vmlinux and memory footprint increase of vmlinux caused by builtin-but-
not-enabled LSMs remains. If it becomes possible to make LSMs dynamically
appendable after boot using loadable kernel modules, these problems will
go away.

Another of concerns for enabling TOMOYO in prebuilt kernels is that who can
provide support when distributor cannot provide support. Due to "those who
compiled kernel code is expected to provide support for that kernel code"
spell, TOMOYO is failing to get enabled in Fedora distribution [1]. The
point of loadable kernel module is to share the workload. If it becomes
possible to make LSMs dynamically appendable after boot using loadable
kernel modules, as with people can use device drivers not supported by
distributors but provided by third party device vendors, we can break
this spell and can lower the barrier for using TOMOYO.

This patch is intended for demonstrating that there is nothing difficult
for supporting TOMOYO-like loadable LSM modules. For now we need to live
with a mixture of built-in part and loadable part because fully loadable
LSM modules are not supported since Linux 2.6.24 [2] and number of LSMs
which can reserve static call slots is determined at compile time in
Linux 6.12.

Major changes in this patch are described below.
There are no behavior changes as long as TOMOYO is built into vmlinux.

Add CONFIG_SECURITY_TOMOYO_LKM as "bool" instead of changing
CONFIG_SECURITY_TOMOYO from "bool" to "tristate", for something went
wrong with how Makefile is evaluated if I choose "tristate".

Add proxy.c for serving as a bridge between vmlinux and tomoyo.ko .
Move callback functions from init.c to proxy.c when building as a loadable
LSM module. init.c is built-in part and remains for reserving static call
slots. proxy.c contains module's init function and tells init.c location of
callback functions, making it possible to use static call for tomoyo.ko .

By deferring initialization of "struct tomoyo_task" until tomoyo.ko is
loaded, threads created between init.c reserved LSM hooks and proxy.c
updates LSM hooks will have NULL "struct tomoyo_task" instances. Assuming
that tomoyo.ko is loaded by the moment when the global init process starts,
initialize "struct tomoyo_task" instance for current thread as a kernel
thread when tomoyo_task(current) is called for the first time.

There is a hack for exporting currently not-exported functions.
This hack will be removed after all relevant functions are exported.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=542986 [1]
Link: https://lkml.kernel.org/r/[email protected] [2]
Signed-off-by: Tetsuo Handa <[email protected]>

Author: Tetsuo Handa

security/tomoyo/Kconfig

@@ -13,6 +13,21 @@ config SECURITY_TOMOYO
 	  found at <https://tomoyo.sourceforge.net/>.
 	  If you are unsure how to answer this question, answer N.
 
+config SECURITY_TOMOYO_LKM
+	bool "Cut out most of TOMOYO's code to a loadable kernel module"
+	default n
+	depends on SECURITY_TOMOYO
+	depends on MODULES
+	help
+	  Say Y here if you want to include TOMOYO without bloating
+	  vmlinux file. If you say Y, most of TOMOYO code is cut out to
+	  a loadable kernel module named tomoyo.ko . This option will be
+	  useful for kernels built by Linux distributors where TOMOYO is
+	  included but TOMOYO is not enabled by default. Please be sure
+	  to explicitly load tomoyo.ko if you want to activate TOMOYO
+	  without calling userspace policy loader, for tomoyo.ko is
+	  loaded immediately before calling userspace policy loader.
+
 config SECURITY_TOMOYO_MAX_ACCEPT_ENTRY
 	int "Default maximal count for learning mode"
 	default 2048

security/tomoyo/Makefile

@@ -1,5 +1,11 @@
 # SPDX-License-Identifier: GPL-2.0
-obj-y = audit.o common.o condition.o domain.o environ.o file.o gc.o group.o init.o load_policy.o memory.o mount.o network.o realpath.o securityfs_if.o util.o
+tomoyo-objs := audit.o common.o condition.o domain.o environ.o file.o gc.o group.o memory.o mount.o network.o proxy.o realpath.o securityfs_if.o util.o
+obj-y += init.o load_policy.o
+ifdef CONFIG_SECURITY_TOMOYO_LKM
+obj-m += tomoyo.o
+else
+obj-y += tomoyo.o
+endif
 
 targets += builtin-policy.h
 

security/tomoyo/common.c

@@ -998,8 +998,13 @@ static bool tomoyo_select_domain(struct tomoyo_io_buffer *head,
 			p = find_task_by_pid_ns(pid, &init_pid_ns);
 		else
 			p = find_task_by_vpid(pid);
-		if (p)
+		if (p) {
 			domain = tomoyo_task(p)->domain_info;
+#ifdef CONFIG_SECURITY_TOMOYO_LKM
+			if (!domain)
+				domain = &tomoyo_kernel_domain;
+#endif
+		}
 		rcu_read_unlock();
 	} else if (!strncmp(data, "domain=", 7)) {
 		if (tomoyo_domain_def(data + 7))
@@ -1710,8 +1715,13 @@ static void tomoyo_read_pid(struct tomoyo_io_buffer *head)
 		p = find_task_by_pid_ns(pid, &init_pid_ns);
 	else
 		p = find_task_by_vpid(pid);
-	if (p)
+	if (p) {
 		domain = tomoyo_task(p)->domain_info;
+#ifdef CONFIG_SECURITY_TOMOYO_LKM
+		if (!domain)
+			domain = &tomoyo_kernel_domain;
+#endif
+	}
 	rcu_read_unlock();
 	if (!domain)
 		return;

security/tomoyo/common.h

@@ -978,6 +978,7 @@ int tomoyo_get_mode(const struct tomoyo_policy_namespace *ns, const u8 profile,
 int tomoyo_init_request_info(struct tomoyo_request_info *r,
 			     struct tomoyo_domain_info *domain,
 			     const u8 index);
+int __init tomoyo_interface_init(void);
 int tomoyo_mkdev_perm(const u8 operation, const struct path *path,
 		      const unsigned int mode, unsigned int dev);
 int tomoyo_mount_permission(const char *dev_name, const struct path *path,
@@ -1214,10 +1215,14 @@ static inline void tomoyo_put_group(struct tomoyo_group *group)
  *
  * Returns pointer to "struct tomoyo_task" for specified thread.
  */
+#ifdef CONFIG_SECURITY_TOMOYO_LKM
+extern struct tomoyo_task *tomoyo_task(struct task_struct *task);
+#else
 static inline struct tomoyo_task *tomoyo_task(struct task_struct *task)
 {
 	return task->security + tomoyo_blob_sizes.lbs_task;
 }
+#endif
 
 /**
  * tomoyo_same_name_union - Check for duplicated "struct tomoyo_name_union" entry.
@@ -1284,4 +1289,71 @@ static inline struct tomoyo_policy_namespace *tomoyo_current_namespace(void)
 		pos =  srcu_dereference((head)->next, &tomoyo_ss);	\
 	for ( ; pos != (head); pos = srcu_dereference(pos->next, &tomoyo_ss))
 
+#ifdef CONFIG_SECURITY_TOMOYO_LKM
+
+#define LSM_HOOK(RET, DEFAULT, NAME, ...) typedef RET (NAME##_t)(__VA_ARGS__);
+#include <linux/lsm_hook_defs.h>
+#undef LSM_HOOK
+
+struct tomoyo_hooks {
+	cred_prepare_t *cred_prepare;
+	bprm_committed_creds_t *bprm_committed_creds;
+	task_alloc_t *task_alloc;
+	task_free_t *task_free;
+	bprm_check_security_t *bprm_check_security;
+	file_fcntl_t *file_fcntl;
+	file_open_t *file_open;
+	file_truncate_t *file_truncate;
+	path_truncate_t *path_truncate;
+	path_unlink_t *path_unlink;
+	path_mkdir_t *path_mkdir;
+	path_rmdir_t *path_rmdir;
+	path_symlink_t *path_symlink;
+	path_mknod_t *path_mknod;
+	path_link_t *path_link;
+	path_rename_t *path_rename;
+	inode_getattr_t *inode_getattr;
+	file_ioctl_t *file_ioctl;
+	file_ioctl_compat_t *file_ioctl_compat;
+	path_chmod_t *path_chmod;
+	path_chown_t *path_chown;
+	path_chroot_t *path_chroot;
+	sb_mount_t *sb_mount;
+	sb_umount_t *sb_umount;
+	sb_pivotroot_t *sb_pivotroot;
+	socket_bind_t *socket_bind;
+	socket_connect_t *socket_connect;
+	socket_listen_t *socket_listen;
+	socket_sendmsg_t *socket_sendmsg;
+};
+
+extern void tomoyo_register_hooks(const struct tomoyo_hooks *tomoyo_hooks);
+
+struct tomoyo_operations {
+	void (*check_profile)(void);
+	int enabled;
+};
+
+extern struct tomoyo_operations tomoyo_ops;
+
+/*
+ * Temporary hack: functions needed by tomoyo.ko . This will be removed
+ * after all functions are marked as EXPORT_STMBOL_GPL().
+ */
+struct tomoyo_tmp_exports {
+	struct task_struct * (*find_task_by_vpid)(pid_t nr);
+	struct task_struct * (*find_task_by_pid_ns)(pid_t nr, struct pid_namespace *ns);
+	void (*put_filesystem)(struct file_system_type *fs);
+	struct file * (*get_mm_exe_file)(struct mm_struct *mm);
+	char * (*d_absolute_path)(const struct path *path, char *buf, int buflen);
+};
+extern const struct tomoyo_tmp_exports tomoyo_tmp_exports;
+#define find_task_by_vpid tomoyo_tmp_exports.find_task_by_vpid
+#define find_task_by_pid_ns tomoyo_tmp_exports.find_task_by_pid_ns
+#define put_filesystem tomoyo_tmp_exports.put_filesystem
+#define get_mm_exe_file tomoyo_tmp_exports.get_mm_exe_file
+#define d_absolute_path tomoyo_tmp_exports.d_absolute_path
+
+#endif /* defined(CONFIG_SECURITY_TOMOYO_LKM) */
+
 #endif /* !defined(_SECURITY_TOMOYO_COMMON_H) */