linux-kernel - Re: [PATCH] proc: prevent a task from writing on its own /proc/*/mem

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Message-ID: <df12287e-7c19-61a6-b94a-5b7186a1e2a3@schaufler-ca.com>
Date:   Sat, 26 May 2018 10:53:04 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Salvatore Mesoraca <s.mesoraca16@...il.com>,
        Alexey Dobriyan <adobriyan@...il.com>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Akinobu Mita <akinobu.mita@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        Davidlohr Bueso <dave@...olabs.net>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] proc: prevent a task from writing on its own /proc/*/mem

On 5/26/2018 10:30 AM, Salvatore Mesoraca wrote:
> 2018-05-26 17:48 GMT+02:00 Alexey Dobriyan <adobriyan@...il.com>:
>> On Sat, May 26, 2018 at 04:50:46PM +0200, Salvatore Mesoraca wrote:
>>> Prevent a task from opening, in "write" mode, any /proc/*/mem
>>> file that operates on the task's mm.
>>> /proc/*/mem is mainly a debugging means and, as such, it shouldn't
>>> be used by the inspected process itself.
>>> Current implementation always allow a task to access its own
>>> /proc/*/mem file.
>>> A process can use it to overwrite read-only memory, making
>>> pointless the use of security_file_mprotect() or other ways to
>>> enforce RO memory.
>> You can do it in security_ptrace_access_check()
> No, because that hook is skipped when mm == current->mm:
> https://elixir.bootlin.com/linux/v4.17-rc6/source/kernel/fork.c#L1111
>
>> or security_file_open()
> This is true, but it looks a bit overkill to me, especially since many of
> the macros/functions used to handle proc's files won't be in scope
> for an external LSM.
> Is there any particular reason why you prefer it done via LSM?

If you did a Yama style LSM it would be easy to configure.
Even though it might make no sense to allow this behavior,
someone, somewhere is counting on it.

>
> Thank you,
>
> Salvatore
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