Messages in this thread Patch in this message | | | From | Luis Henriques <> | Subject | [PATCH 114/118] exec: do not abuse ->cred_guard_mutex in threadgroup_lock() | Date | Tue, 7 May 2013 14:39:20 +0100 |
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3.5.7.12 -stable review patch. If anyone has any objections, please let me know.
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From: Oleg Nesterov <oleg@redhat.com>
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(-)
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) {} -- 1.8.1.2
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