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 <da...@redhat.com>
Acked-by: Tejun Heo <t...@kernel.org>
Acked-by: Li Zefan <lize...@huawei.com>
Signed-off-by: Oleg Nesterov <o...@redhat.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 20df02c..bea2f7d 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -898,11 +898,13 @@ 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_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 *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 932a90c..67cfdb5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2486,27 +2486,18 @@ 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);
}
@@ -2519,7 +2510,6 @@ 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) {}
--
1.5.5.1
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