LSM: Revive security_task_alloc() hook and per "struct task_struct" security blob.

We switched from "struct task_struct"->security to "struct cred"->security
in Linux 2.6.29. But not all LSM modules were happy with that change.
TOMOYO LSM module is an example which want to use per "struct task_struct"
security blob, for TOMOYO's security context is defined based on "struct
task_struct" rather than "struct cred". AppArmor LSM module is another
example which want to use it, for AppArmor is currently abusing the cred
a little bit to store the change_hat and setexeccon info. Although
security_task_free() hook was revived in Linux 3.4 because Yama LSM module
wanted to release per "struct task_struct" security blob,
security_task_alloc() hook and "struct task_struct"->security field were
not revived. Nowadays, we are getting proposals of lightweight LSM modules
which want to use per "struct task_struct" security blob.

We are already allowing multiple concurrent LSM modules (up to one fully
armored module which uses "struct cred"->security field or exclusive hooks
like security_xfrm_state_pol_flow_match(), plus unlimited number of
lightweight modules which do not use "struct cred"->security nor exclusive
hooks) as long as they are built into the kernel. But this patch does not
implement variable length "struct task_struct"->security field which will
become needed when multiple LSM modules want to use "struct task_struct"->
security field. Although it won't be difficult to implement variable length
"struct task_struct"->security field, let's think about it after we merged
this patch.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Tested-by: Djalal Harouni <tixxdz@gmail.com>
Acked-by: José Bollo <jobol@nonadev.net>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: Eric Paris <eparis@parisplace.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: James Morris <james.l.morris@oracle.com>
Cc: José Bollo <jobol@nonadev.net>
Signed-off-by: James Morris <james.l.morris@oracle.com>

include/linux/init_task.h

@@ -210,6 +210,12 @@ extern struct cred init_cred;
 # define INIT_TASK_TI(tsk)
 #endif
 
+#ifdef CONFIG_SECURITY
+#define INIT_TASK_SECURITY .security = NULL,
+#else
+#define INIT_TASK_SECURITY
+#endif
+
 /*
  *  INIT_TASK is used to set up the first task table, touch at
  * your own risk!. Base=0, limit=0x1fffff (=2MB)
@@ -288,6 +294,7 @@ extern struct cred init_cred;
 	INIT_VTIME(tsk)							\
 	INIT_NUMA_BALANCING(tsk)					\
 	INIT_KASAN(tsk)							\
+	INIT_TASK_SECURITY						\
 }
 
 

include/linux/lsm_hooks.h

@@ -533,8 +533,13 @@
  *	manual page for definitions of the @clone_flags.
  *	@clone_flags contains the flags indicating what should be shared.
  *	Return 0 if permission is granted.
+ * @task_alloc:
+ *	@task task being allocated.
+ *	@clone_flags contains the flags indicating what should be shared.
+ *	Handle allocation of task-related resources.
+ *	Returns a zero on success, negative values on failure.
  * @task_free:
- *	@task task being freed
+ *	@task task about to be freed.
  *	Handle release of task-related resources. (Note that this can be called
  *	from interrupt context.)
  * @cred_alloc_blank:
@@ -1482,6 +1487,7 @@ union security_list_options {
 	int (*file_open)(struct file *file, const struct cred *cred);
 
 	int (*task_create)(unsigned long clone_flags);
+	int (*task_alloc)(struct task_struct *task, unsigned long clone_flags);
 	void (*task_free)(struct task_struct *task);
 	int (*cred_alloc_blank)(struct cred *cred, gfp_t gfp);
 	void (*cred_free)(struct cred *cred);
@@ -1748,6 +1754,7 @@ struct security_hook_heads {
 	struct list_head file_receive;
 	struct list_head file_open;
 	struct list_head task_create;
+	struct list_head task_alloc;
 	struct list_head task_free;
 	struct list_head cred_alloc_blank;
 	struct list_head cred_free;

include/linux/sched.h

@@ -1037,6 +1037,10 @@ struct task_struct {
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 	/* A live task holds one reference: */
 	atomic_t			stack_refcount;
+#endif
+#ifdef CONFIG_SECURITY
+	/* Used by LSM modules for access restriction: */
+	void				*security;
 #endif
 	/* CPU-specific state of this task: */
 	struct thread_struct		thread;

include/linux/security.h

@@ -308,6 +308,7 @@ int security_file_send_sigiotask(struct task_struct *tsk,
 int security_file_receive(struct file *file);
 int security_file_open(struct file *file, const struct cred *cred);
 int security_task_create(unsigned long clone_flags);
+int security_task_alloc(struct task_struct *task, unsigned long clone_flags);
 void security_task_free(struct task_struct *task);
 int security_cred_alloc_blank(struct cred *cred, gfp_t gfp);
 void security_cred_free(struct cred *cred);
@@ -861,6 +862,12 @@ static inline int security_task_create(unsigned long clone_flags)
 	return 0;
 }
 
+static inline int security_task_alloc(struct task_struct *task,
+				      unsigned long clone_flags)
+{
+	return 0;
+}
+
 static inline void security_task_free(struct task_struct *task)
 { }
 

kernel/fork.c

@@ -1679,9 +1679,12 @@ static __latent_entropy struct task_struct *copy_process(
 		goto bad_fork_cleanup_perf;
 	/* copy all the process information */
 	shm_init_task(p);
-	retval = copy_semundo(clone_flags, p);
+	retval = security_task_alloc(p, clone_flags);
 	if (retval)
 		goto bad_fork_cleanup_audit;
+	retval = copy_semundo(clone_flags, p);
+	if (retval)
+		goto bad_fork_cleanup_security;
 	retval = copy_files(clone_flags, p);
 	if (retval)
 		goto bad_fork_cleanup_semundo;
@@ -1903,6 +1906,8 @@ static __latent_entropy struct task_struct *copy_process(
 	exit_files(p); /* blocking */
 bad_fork_cleanup_semundo:
 	exit_sem(p);
+bad_fork_cleanup_security:
+	security_task_free(p);
 bad_fork_cleanup_audit:
 	audit_free(p);
 bad_fork_cleanup_perf:

security/security.c

@@ -937,6 +937,11 @@ int security_task_create(unsigned long clone_flags)
 	return call_int_hook(task_create, 0, clone_flags);
 }
 
+int security_task_alloc(struct task_struct *task, unsigned long clone_flags)
+{
+	return call_int_hook(task_alloc, 0, task, clone_flags);
+}
+
 void security_task_free(struct task_struct *task)
 {
 	call_void_hook(task_free, task);