linux-kernel - Re: [PATCH] fs/exec.c: Avoid a race in formats

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Date:   Wed, 23 Feb 2022 18:41:53 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Yun Levi <ppbuk5246@...il.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs/exec.c: Avoid a race in formats

Yun Levi <ppbuk5246@...il.com> writes:

> On Thu, Feb 24, 2022 at 8:59 AM Yun Levi <ppbuk5246@...il.com> wrote:
>>
>> On Thu, Feb 24, 2022 at 8:24 AM Al Viro <viro@...iv.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.

Mostly of what has been happening with binary formats lately is code
removal.

So I humbly suggest the best defense against misuse by modules is to
simply remove "EXPORT_SYMBOL(__register_binfmt)".

Eric