[PATCH] umh: fix UAF when the process is being killed

[Schspa Shi <schspa@gmail.com>]

When the process is killed, wait_for_completion_state will return with
-ERESTARTSYS, and the completion variable in the stack will be freed.
If the user-mode thread is complete at the same time, there will be a UAF.

Please refer to the following scenarios.
            T1                                  T2
------------------------------------------------------------------
call_usermodehelper_exec
                                   call_usermodehelper_exec_async
                                   << do something >>
                                   umh_complete(sub_info);
                                   comp = xchg(&sub_info->complete, NULL);
                                   /* we got the completion */
                                   << context switch >>

    << Being killed >>
	retval = wait_for_completion_state(sub_info->complete, state);
	if (!retval)
		goto wait_done;

	if (wait & UMH_KILLABLE) {
		/* umh_complete() will see NULL and free sub_info */
		if (xchg(&sub_info->complete, NULL))
			goto unlock;
        << we can't got the completion >>
	}
	....
unlock:
	helper_unlock();
	return retval;
}

/**
 * the completion variable in stack is end of life cycle.
 * and maybe freed due to process is recycled.
 */
                                   --------UAF here----------
                                   if (comp)
                                       complete(comp);

To fix it, we can put the completion variable in the subprocess_info
variable.

Reported-by: syzbot+10d19d528d9755d9af22@syzkaller.appspotmail.com
Reported-by: syzbot+70d5d5d83d03db2c813d@syzkaller.appspotmail.com
Reported-by: syzbot+83cb0411d0fcf0a30fc1@syzkaller.appspotmail.com

Signed-off-by: Schspa Shi <schspa@gmail.com>
---
 include/linux/umh.h | 1 +
 kernel/umh.c        | 6 +++---
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/umh.h b/include/linux/umh.h
index 5d1f6129b847..801f7efbc825 100644
--- a/include/linux/umh.h
+++ b/include/linux/umh.h
@@ -20,6 +20,7 @@ struct file;
 struct subprocess_info {
 	struct work_struct work;
 	struct completion *complete;
+	struct completion done;
 	const char *path;
 	char **argv;
 	char **envp;
diff --git a/kernel/umh.c b/kernel/umh.c
index 850631518665..3ed39956c777 100644
--- a/kernel/umh.c
+++ b/kernel/umh.c
@@ -380,6 +380,7 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
 	sub_info->cleanup = cleanup;
 	sub_info->init = init;
 	sub_info->data = data;
+	init_completion(&sub_info->done);
   out:
 	return sub_info;
 }
@@ -405,7 +406,6 @@ EXPORT_SYMBOL(call_usermodehelper_setup);
 int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 {
 	unsigned int state = TASK_UNINTERRUPTIBLE;
-	DECLARE_COMPLETION_ONSTACK(done);
 	int retval = 0;
 
 	if (!sub_info->path) {
@@ -431,7 +431,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	 * This makes it possible to use umh_complete to free
 	 * the data structure in case of UMH_NO_WAIT.
 	 */
-	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
+	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &sub_info->done;
 	sub_info->wait = wait;
 
 	queue_work(system_unbound_wq, &sub_info->work);
@@ -444,7 +444,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 	if (wait & UMH_FREEZABLE)
 		state |= TASK_FREEZABLE;
 
-	retval = wait_for_completion_state(&done, state);
+	retval = wait_for_completion_state(sub_info->complete, state);
 	if (!retval)
 		goto wait_done;
 
-- 
2.37.3