CRED: Fix __task_cred()'s lockdep check and banner comment

Fix __task_cred()'s lockdep check by removing the following validation
condition:

	lockdep_tasklist_lock_is_held()

as commit_creds() does not take the tasklist_lock, and nor do most of the
functions that call it, so this check is pointless and it can prevent
detection of the RCU lock not being held if the tasklist_lock is held.

Instead, add the following validation condition:

	task->exit_state >= 0

to permit the access if the target task is dead and therefore unable to change
its own credentials.

Fix __task_cred()'s comment to:

 (1) discard the bit that says that the caller must prevent the target task
     from being deleted.  That shouldn't need saying.

 (2) Add a comment indicating the result of __task_cred() should not be passed
     directly to get_cred(), but rather than get_task_cred() should be used
     instead.

Also put a note into the documentation to enforce this point there too.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 11 insertions, 5 deletions

diff --git a/include/linux/cred.h b/include/linux/cred.h
index ce40cbc791e2..4d2c39573f36 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -274,13 +274,18 @@ static inline void put_cred(const struct cred *_cred)
  * @task: The task to query
  *
  * Access the objective credentials of a task.  The caller must hold the RCU
- * readlock.
+ * readlock or the task must be dead and unable to change its own credentials.
  *
- * 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.
+ * The result of this function should not be passed directly to get_cred();
+ * rather get_task_cred() should be used instead.
  */
-#define __task_cred(task) \
-        ((const struct cred *)(rcu_dereference_check((task)->real_cred, rcu_read_lock_held() || lockdep_tasklist_lock_is_held())))
+#define __task_cred(task)                                               \
+        ({                                                              \
+                const struct task_struct *__t = (task);                 \
+                rcu_dereference_check(__t->real_cred,                   \
+                                      rcu_read_lock_held() ||           \
+                                      task_is_dead(__t));               \
+        })
 
 /**
  * get_current_cred - Get the current task's subjective credentials
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 747fcaedddb7..0478888c6899 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -214,6 +214,7 @@ extern char ___assert_task_state[1 - 2*!!(
 
 #define task_is_traced(task)    ((task->state & __TASK_TRACED) != 0)
 #define task_is_stopped(task)   ((task->state & __TASK_STOPPED) != 0)
+#define task_is_dead(task)      ((task)->exit_state != 0)
 #define task_is_stopped_or_traced(task) \
                         ((task->state & (__TASK_STOPPED | __TASK_TRACED)) != 0)
 #define task_contributes_to_load(task)  \