CRED: Differentiate objective and effective subjective credentials on a task

Differentiate the objective and real subjective credentials from the effective
subjective credentials on a task by introducing a second credentials pointer
into the task_struct.

task_struct::real_cred then refers to the objective and apparent real
subjective credentials of a task, as perceived by the other tasks in the
system.

task_struct::cred then refers to the effective subjective credentials of a
task, as used by that task when it's actually running.  These are not visible
to the other tasks in the system.

__task_cred(task) then refers to the objective/real credentials of the task in
question.

current_cred() refers to the effective subjective credentials of the current
task.

prepare_creds() uses the objective creds as a base and commit_creds() changes
both pointers in the task_struct (indeed commit_creds() requires them to be the
same).

override_creds() and revert_creds() change the subjective creds pointer only,
and the former returns the old subjective creds.  These are used by NFSD,
faccessat() and do_coredump(), and will by used by CacheFiles.

In SELinux, current_has_perm() is provided as an alternative to
task_has_perm().  This uses the effective subjective context of current,
whereas task_has_perm() uses the objective/real context of the subject.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

3 files changed, 20 insertions, 15 deletions

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 794aab5c66e5..55a9c995d694 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -146,8 +146,8 @@ extern struct cred *prepare_exec_creds(void);
 extern struct cred *prepare_usermodehelper_creds(void);
 extern int commit_creds(struct cred *);
 extern void abort_creds(struct cred *);
-extern const struct cred *override_creds(const struct cred *) __deprecated;
-extern void revert_creds(const struct cred *) __deprecated;
+extern const struct cred *override_creds(const struct cred *);
+extern void revert_creds(const struct cred *);
 extern void __init cred_init(void);
 
 /**
@@ -202,32 +202,32 @@ static inline void put_cred(const struct cred *_cred)
 }
 
 /**
- * current_cred - Access the current task's credentials
+ * current_cred - Access the current task's subjective credentials
  *
- * Access the credentials of the current task.
+ * Access the subjective credentials of the current task.
  */
 #define current_cred() \
 	(current->cred)
 
 /**
- * __task_cred - Access another task's credentials
+ * __task_cred - Access a task's objective credentials
  * @task: The task to query
  *
- * Access the credentials of another task.  The caller must hold the
- * RCU readlock.
+ * Access the objective credentials of a task.  The caller must hold the RCU
+ * readlock.
  *
  * The caller must make sure task doesn't go away, either by holding a ref on
  * task or by holding tasklist_lock to prevent it from being unlinked.
  */
 #define __task_cred(task) \
-	((const struct cred *)(rcu_dereference((task)->cred)))
+	((const struct cred *)(rcu_dereference((task)->real_cred)))
 
 /**
- * get_task_cred - Get another task's credentials
+ * get_task_cred - Get another task's objective credentials
  * @task: The task to query
  *
- * Get the credentials of a task, pinning them so that they can't go away.
- * Accessing a task's credentials directly is not permitted.
+ * Get the objective credentials of a task, pinning them so that they can't go
+ * away.  Accessing a task's credentials directly is not permitted.
  *
  * The caller must make sure task doesn't go away, either by holding a ref on
  * task or by holding tasklist_lock to prevent it from being unlinked.
@@ -243,10 +243,11 @@ static inline void put_cred(const struct cred *_cred)
 })
 
 /**
- * get_current_cred - Get the current task's credentials
+ * get_current_cred - Get the current task's subjective credentials
  *
- * Get the credentials of the current task, pinning them so that they can't go
- * away.  Accessing the current task's credentials directly is not permitted.
+ * Get the subjective credentials of the current task, pinning them so that
+ * they can't go away.  Accessing the current task's credentials directly is
+ * not permitted.
  */
 #define get_current_cred()				\
 	(get_cred(current_cred()))
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 08c3b24ad9a8..2597858035cd 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -149,6 +149,7 @@ extern struct cred init_cred;
 	.children	= LIST_HEAD_INIT(tsk.children),			\
 	.sibling	= LIST_HEAD_INIT(tsk.sibling),			\
 	.group_leader	= &tsk,						\
+	.real_cred	= &init_cred,					\
 	.cred		= &init_cred,					\
 	.cred_exec_mutex =						\
 		 __MUTEX_INITIALIZER(tsk.cred_exec_mutex),		\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 121d655e460d..3443123b0709 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1145,7 +1145,10 @@ struct task_struct {
 	struct list_head cpu_timers[3];
 
 /* process credentials */
-	const struct cred *cred;	/* actual/objective task credentials (COW) */
+	const struct cred *real_cred;	/* objective and real subjective task
+					 * credentials (COW) */
+	const struct cred *cred;	/* effective (overridable) subjective task
+					 * credentials (COW) */
 	struct mutex cred_exec_mutex;	/* execve vs ptrace cred calculation mutex */
 
 	char comm[TASK_COMM_LEN]; /* executable name excluding path