Patch "exec: do not abuse ->cred_guard_mutex in threadgroup_lock()" has been added to the 3.9-stable tree

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This is a note to let you know that I've just added the patch titled

    exec: do not abuse ->cred_guard_mutex in threadgroup_lock()

to the 3.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     exec-do-not-abuse-cred_guard_mutex-in-threadgroup_lock.patch
and it can be found in the queue-3.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From e56fb2874015370e3b7f8d85051f6dce26051df9 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Date: Tue, 30 Apr 2013 15:28:20 -0700
Subject: exec: do not abuse ->cred_guard_mutex in threadgroup_lock()

From: Oleg Nesterov <oleg@xxxxxxxxxx>

commit e56fb2874015370e3b7f8d85051f6dce26051df9 upstream.

threadgroup_lock() takes signal->cred_guard_mutex to ensure that
thread_group_leader() is stable.  This doesn't look nice, the scope of
this lock in do_execve() is huge.

And as Dave pointed out this can lead to deadlock, we have the
following dependencies:

	do_execve:		cred_guard_mutex -> i_mutex
	cgroup_mount:		i_mutex -> cgroup_mutex
	attach_task_by_pid:	cgroup_mutex -> cred_guard_mutex

Change de_thread() to take threadgroup_change_begin() around the
switch-the-leader code and change threadgroup_lock() to avoid
->cred_guard_mutex.

Note that de_thread() can't sleep with ->group_rwsem held, this can
obviously deadlock with the exiting leader if the writer is active, so it
does threadgroup_change_end() before schedule().

Reported-by: Dave Jones <davej@xxxxxxxxxx>
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Acked-by: Li Zefan <lizefan@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/exec.c             |    3 +++
 include/linux/sched.h |   18 ++++--------------
 2 files changed, 7 insertions(+), 14 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -898,11 +898,13 @@ static int de_thread(struct task_struct
 
 		sig->notify_count = -1;	/* for exit_notify() */
 		for (;;) {
+			threadgroup_change_begin(tsk);
 			write_lock_irq(&tasklist_lock);
 			if (likely(leader->exit_state))
 				break;
 			__set_current_state(TASK_KILLABLE);
 			write_unlock_irq(&tasklist_lock);
+			threadgroup_change_end(tsk);
 			schedule();
 			if (unlikely(__fatal_signal_pending(tsk)))
 				goto killed;
@@ -960,6 +962,7 @@ static int de_thread(struct task_struct
 		if (unlikely(leader->ptrace))
 			__wake_up_parent(leader, leader->parent);
 		write_unlock_irq(&tasklist_lock);
+		threadgroup_change_end(tsk);
 
 		release_task(leader);
 	}
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2413,27 +2413,18 @@ static inline void threadgroup_change_en
  *
  * Lock the threadgroup @tsk belongs to.  No new task is allowed to enter
  * and member tasks aren't allowed to exit (as indicated by PF_EXITING) or
- * perform exec.  This is useful for cases where the threadgroup needs to
- * stay stable across blockable operations.
+ * change ->group_leader/pid.  This is useful for cases where the threadgroup
+ * needs to stay stable across blockable operations.
  *
  * fork and exit paths explicitly call threadgroup_change_{begin|end}() for
  * synchronization.  While held, no new task will be added to threadgroup
  * and no existing live task will have its PF_EXITING set.
  *
- * During exec, a task goes and puts its thread group through unusual
- * changes.  After de-threading, exclusive access is assumed to resources
- * which are usually shared by tasks in the same group - e.g. sighand may
- * be replaced with a new one.  Also, the exec'ing task takes over group
- * leader role including its pid.  Exclude these changes while locked by
- * grabbing cred_guard_mutex which is used to synchronize exec path.
+ * de_thread() does threadgroup_change_{begin|end}() when a non-leader
+ * sub-thread becomes a new leader.
  */
 static inline void threadgroup_lock(struct task_struct *tsk)
 {
-	/*
-	 * exec uses exit for de-threading nesting group_rwsem inside
-	 * cred_guard_mutex. Grab cred_guard_mutex first.
-	 */
-	mutex_lock(&tsk->signal->cred_guard_mutex);
 	down_write(&tsk->signal->group_rwsem);
 }
 
@@ -2446,7 +2437,6 @@ static inline void threadgroup_lock(stru
 static inline void threadgroup_unlock(struct task_struct *tsk)
 {
 	up_write(&tsk->signal->group_rwsem);
-	mutex_unlock(&tsk->signal->cred_guard_mutex);
 }
 #else
 static inline void threadgroup_change_begin(struct task_struct *tsk) {}


Patches currently in stable-queue which might be from oleg@xxxxxxxxxx are

queue-3.9/exec-do-not-abuse-cred_guard_mutex-in-threadgroup_lock.patch
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