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From:   Salvatore Mesoraca <s.mesoraca16@gmail.com>
Date:   Sat, 26 May 2018 19:30:47 +0200
Message-ID: <CAJHCu1LgSUJdiZEfParCH7aLERWM1bgwC7e8wQKgmkNE01_4KA@mail.gmail.com>
Subject: Re: [PATCH] proc: prevent a task from writing on its own /proc/*/mem
To:     Alexey Dobriyan <adobriyan@gmail.com>
Cc:     Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Akinobu Mita <akinobu.mita@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Kees Cook <keescook@chromium.org>
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2018-05-26 17:48 GMT+02:00 Alexey Dobriyan <adobriyan@gmail.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?

Thank you,

Salvatore