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From:   Yun Levi <ppbuk5246@gmail.com>
Date:   Thu, 24 Feb 2022 09:10:43 +0900
Message-ID: <CAM7-yPSk35UoGmRY_rCo2=RryBvwbQEjeWfL2tz1ADUosCXNjw@mail.gmail.com>
Subject: Re: [PATCH] fs/exec.c: Avoid a race in formats
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Kees Cook <keescook@chromium.org>, ebiederm@xmission.com,
        linux-fsdevel@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, Feb 24, 2022 at 8:59 AM Yun Levi <ppbuk5246@gmail.com> wrote:
>
> On Thu, Feb 24, 2022 at 8:24 AM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > On Thu, Feb 24, 2022 at 08:17:52AM +0900, Levi Yun wrote:
> > > Suppose a module registers its own binfmt (custom) and formats is like:
> > >
> > > +---------+    +----------+    +---------+
> > > | custom  | -> |  format1 | -> | format2 |
> > > +---------+    +----------+    +---------+
> > >
> > > and try to call unregister_binfmt with custom NOT in __exit stage.
> >
> > Explain, please.  Why would anyone do that?  And how would such
> > module decide when it's safe to e.g. dismantle data structures
> > used by methods of that binfmt, etc.?
> > Could you give more detailed example?
>
> I think if someone wants to control their own binfmt via "ioctl" not
> on time on LOAD.
> For example, someone wants to control exec (notification,
> allow/disallow and etc..)
> and want to enable and disable own's control exec via binfmt reg / unreg
> In that situation, While the module is loaded, binfmt is still live
> and can be reused by
> reg/unreg to enable/disable his exec' control.
>
> module can decide it's safe to unload by tracing the stack and
> confirming whether some tasks in the custom binfmt's function after it
> unregisters its own binfmt.
>
> > Because it looks like papering over an inherently unsafe use of binfmt interfaces..
>
> I think the above example it's quite a trick and stupid.  it's quite
> unsafe to use as you mention.
> But, misuse allows that situation to happen without any warning.
> As a robustness, I just try to avoid above situation But,
> I think it's better to restrict unregister binfmt unregister only when
> there is no module usage.

And not only stupid exmaple,
if someone loadable custom binfmt register in __init and __exit via
register and unregister_binfmt,
I think that situation could happen.