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 <[email protected]>
Signed-off-by: James Morris <[email protected]>

fs/nfsd/auth.c

@@ -34,6 +34,8 @@ int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
 	int flags = nfsexp_flags(rqstp, exp);
 	int ret;
 
+	/* discard any old override before preparing the new set */
+	revert_creds(get_cred(current->real_cred));
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
@@ -82,7 +84,8 @@ int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
 	else
 		new->cap_effective = cap_raise_nfsd_set(new->cap_effective,
 							new->cap_permitted);
-	return commit_creds(new);
+	put_cred(override_creds(new));
+	return 0;
 
 oom:
 	ret = -ENOMEM;

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()))

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),		\

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

kernel/cred.c

@@ -35,7 +35,7 @@ static struct thread_group_cred init_tgcred = {
  * The initial credentials for the initial task
  */
 struct cred init_cred = {
-	.usage			= ATOMIC_INIT(3),
+	.usage			= ATOMIC_INIT(4),
 	.securebits		= SECUREBITS_DEFAULT,
 	.cap_inheritable	= CAP_INIT_INH_SET,
 	.cap_permitted		= CAP_FULL_SET,
@@ -120,6 +120,8 @@ EXPORT_SYMBOL(__put_cred);
  * prepare a new copy, which the caller then modifies and then commits by
  * calling commit_creds().
  *
+ * Preparation involves making a copy of the objective creds for modification.
+ *
  * Returns a pointer to the new creds-to-be if successful, NULL otherwise.
  *
  * Call commit_creds() or abort_creds() to clean up.
@@ -130,7 +132,7 @@ struct cred *prepare_creds(void)
 	const struct cred *old;
 	struct cred *new;
 
-	BUG_ON(atomic_read(&task->cred->usage) < 1);
+	BUG_ON(atomic_read(&task->real_cred->usage) < 1);
 
 	new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
 	if (!new)
@@ -262,6 +264,9 @@ struct cred *prepare_usermodehelper_creds(void)
  *
  * We share if we can, but under some circumstances we have to generate a new
  * set.
+ *
+ * The new process gets the current process's subjective credentials as its
+ * objective and subjective credentials
  */
 int copy_creds(struct task_struct *p, unsigned long clone_flags)
 {
@@ -278,6 +283,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
 		clone_flags & CLONE_THREAD
 	    ) {
+		p->real_cred = get_cred(p->cred);
 		get_cred(p->cred);
 		atomic_inc(&p->cred->user->processes);
 		return 0;
@@ -317,7 +323,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
 
 	atomic_inc(&new->user->processes);
-	p->cred = new;
+	p->cred = p->real_cred = get_cred(new);
 	return 0;
 }
 
@@ -326,7 +332,9 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
  * @new: The credentials to be assigned
  *
  * Install a new set of credentials to the current task, using RCU to replace
- * the old set.
+ * the old set.  Both the objective and the subjective credentials pointers are
+ * updated.  This function may not be called if the subjective credentials are
+ * in an overridden state.
  *
  * This function eats the caller's reference to the new credentials.
  *
@@ -338,12 +346,15 @@ int commit_creds(struct cred *new)
 	struct task_struct *task = current;
 	const struct cred *old;
 
+	BUG_ON(task->cred != task->real_cred);
+	BUG_ON(atomic_read(&task->real_cred->usage) < 2);
 	BUG_ON(atomic_read(&new->usage) < 1);
-	BUG_ON(atomic_read(&task->cred->usage) < 1);
 
-	old = task->cred;
+	old = task->real_cred;
 	security_commit_creds(new, old);
 
+	get_cred(new); /* we will require a ref for the subj creds too */
+
 	/* dumpability changes */
 	if (old->euid != new->euid ||
 	    old->egid != new->egid ||
@@ -369,6 +380,7 @@ int commit_creds(struct cred *new)
 	 */
 	if (new->user != old->user)
 		atomic_inc(&new->user->processes);
+	rcu_assign_pointer(task->real_cred, new);
 	rcu_assign_pointer(task->cred, new);
 	if (new->user != old->user)
 		atomic_dec(&old->user->processes);
@@ -388,6 +400,8 @@ int commit_creds(struct cred *new)
 	    new->fsgid != old->fsgid)
 		proc_id_connector(task, PROC_EVENT_GID);
 
+	/* release the old obj and subj refs both */
+	put_cred(old);
 	put_cred(old);
 	return 0;
 }
@@ -408,11 +422,11 @@ void abort_creds(struct cred *new)
 EXPORT_SYMBOL(abort_creds);
 
 /**
- * override_creds - Temporarily override the current process's credentials
+ * override_creds - Override the current process's subjective credentials
  * @new: The credentials to be assigned
  *
- * Install a set of temporary override credentials on the current process,
- * returning the old set for later reversion.
+ * Install a set of temporary override subjective credentials on the current
+ * process, returning the old set for later reversion.
  */
 const struct cred *override_creds(const struct cred *new)
 {
@@ -424,11 +438,11 @@ const struct cred *override_creds(const struct cred *new)
 EXPORT_SYMBOL(override_creds);
 
 /**
- * revert_creds - Revert a temporary credentials override
+ * revert_creds - Revert a temporary subjective credentials override
  * @old: The credentials to be restored
  *
- * Revert a temporary set of override credentials to an old set, discarding the
- * override set.
+ * Revert a temporary set of override subjective credentials to an old set,
+ * discarding the override set.
  */
 void revert_creds(const struct cred *old)
 {

kernel/fork.c

@@ -146,6 +146,7 @@ void __put_task_struct(struct task_struct *tsk)
 	WARN_ON(atomic_read(&tsk->usage));
 	WARN_ON(tsk == current);
 
+	put_cred(tsk->real_cred);
 	put_cred(tsk->cred);
 	delayacct_tsk_free(tsk);
 
@@ -961,10 +962,10 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	DEBUG_LOCKS_WARN_ON(!p->softirqs_enabled);
 #endif
 	retval = -EAGAIN;
-	if (atomic_read(&p->cred->user->processes) >=
+	if (atomic_read(&p->real_cred->user->processes) >=
 			p->signal->rlim[RLIMIT_NPROC].rlim_cur) {
 		if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_RESOURCE) &&
-		    p->cred->user != current->nsproxy->user_ns->root_user)
+		    p->real_cred->user != current->nsproxy->user_ns->root_user)
 			goto bad_fork_free;
 	}
 
@@ -1278,6 +1279,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	module_put(task_thread_info(p)->exec_domain->module);
 bad_fork_cleanup_count:
 	atomic_dec(&p->cred->user->processes);
+	put_cred(p->real_cred);
 	put_cred(p->cred);
 bad_fork_free:
 	free_task(p);