linux-kernel - Re: [PATCH] kernel/kmod: fix use-after-free of the sub

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Date:	Fri, 17 Oct 2014 21:55:15 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	schwidefsky@...ibm.com, oleg@...hat.com
Cc:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

Martin Schwidefsky wrote:
> Oleg Nesterov <oleg@...hat.com> wrote:
> > I also agree that the changelog could mention exec_mmap. Plus a comment
> > about UMH_NO_WAIT && sub_info->complete == NULL. So yes, perhaps v2 makes
> > sense if Martin agrees.
> 
> Version 2 of the patch. All change requests have gone in except for the
> mention of exec_mmap. I don't quite get the relevance of it, do_execve
> can fail for the various reasons.

OK. I understood that the

| For UMH_NO_WAIT __call_usermodehelper() frees the sub_info structure
| directly, for UMH_WAIT_EXEC the call to umh_complete() allows
| call_usermodehelper_exec() to continue which then frees sub_info.

lines described the following sequence.

----------
Race window for call_usermodehelper_exec(UMH_WAIT_EXEC) case is shown below.

(1) The caller enters into sleep at wait_for_completion()
    in call_usermodehelper_exec().
(2) The khelper thread calls __call_usermodehelper() and enters into
    sleep at kthread_create(CLONE_VFORK).
(3) The worker thread calls do_execve() in ____call_usermodehelper()
    called from call_helper().
(4) complete_vfork_done() called from mm_release() called from exec_mmap()
    called from flush_old_exec() called from binfmt handler wakes up
    the khelper thread.
(5) The khelper thread calls umh_complete() and wakes up the caller.
(6) The caller fetches sub_info->retval (which is 0) and calls
    call_usermodehelper_freeinfo().
(7) do_execve() returns a negative value due to an unexpected failure
    after complete_vfork_done() was called.
(8) The worker thread tries to store sub_info->retval (which is a negative
    value).

When hitting this race window, the caller fails to know that do_execve()
failed and the worker thread triggers use-after-free memory corruption.

The race window for call_usermodehelper_exec(UMH_NO_WAIT) case is similar
except that the caller does not enter into sleep at (1) and the khelper
thread calls call_usermodehelper_freeinfo() at (5).
----------

do_execve() can fail for the various reasons. But this race unlikely happens
because do_execve() seldom fails after complete_vfork_done() was called.
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