diff options
authorOleg Nesterov <oleg@redhat.com>2013-04-30 15:28:20 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2013-04-30 17:04:07 -0700
commite56fb2874015370e3b7f8d85051f6dce26051df9 (patch)
parent12eaaf309a798973d215f7f21aa5a67a760ed7c8 (diff)
exec: do not abuse ->cred_guard_mutex in threadgroup_lock()
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> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2 files changed, 7 insertions, 14 deletions
diff --git a/fs/exec.c b/fs/exec.c
index 260f89f66651..963f510a25ab 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);
if (likely(leader->exit_state))
+ threadgroup_change_end(tsk);
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);
+ threadgroup_change_end(tsk);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index a22baf83c20c..6f950048b6e9 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2249,27 +2249,18 @@ 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);
@@ -2282,7 +2273,6 @@ static inline void threadgroup_lock(struct task_struct *tsk)
static inline void threadgroup_unlock(struct task_struct *tsk)
- mutex_unlock(&tsk->signal->cred_guard_mutex);
static inline void threadgroup_change_begin(struct task_struct *tsk) {}