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[3.5.y.z,extended,stable] Patch "exec: do not abuse ->cred_guard_mutex in threadgroup_lock()" has been added to staging queue

Message ID 1367922840-25394-1-git-send-email-luis.henriques@canonical.com
State New
Headers show

Commit Message

Luis Henriques May 7, 2013, 10:34 a.m. UTC 
This is a note to let you know that I have just added a patch titled

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

to the linux-3.5.y-queue branch of the 3.5.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.5.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

From 19e8fd215353399ac84057eee9b39048d4f6956c Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Tue, 30 Apr 2013 15:28:20 -0700
Subject: [PATCH] exec: do not abuse ->cred_guard_mutex in threadgroup_lock()

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@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ luis: adjust context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
---
 fs/exec.c             |  3 +++
 include/linux/sched.h | 18 ++++--------------
 2 files changed, 7 insertions(+), 14 deletions(-)

--
1.8.1.2
diff mbox

Patch

diff --git a/fs/exec.c b/fs/exec.c
index 2013724..5a76464 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -905,11 +905,13 @@  static int de_thread(struct task_struct *tsk)

 		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_UNINTERRUPTIBLE);
 			write_unlock_irq(&tasklist_lock);
+			threadgroup_change_end(tsk);
 			schedule();
 		}

@@ -965,6 +967,7 @@  static int de_thread(struct task_struct *tsk)
 		if (unlikely(leader->ptrace))
 			__wake_up_parent(leader, leader->parent);
 		write_unlock_irq(&tasklist_lock);
+		threadgroup_change_end(tsk);

 		release_task(leader);
 	}
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 3a9e314..ebd15f9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2445,27 +2445,18 @@  static inline void threadgroup_change_end(struct task_struct *tsk)
  *
  * 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);
 }

@@ -2478,7 +2469,6 @@  static inline void threadgroup_lock(struct task_struct *tsk)
 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) {}