linux-kernel - Re: [PATCH v2] exec: Set file unwritable before LSM check

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Message-ID: <CAGXu5j++Ju0Jg0O3gXPGD7Nzy8uE3NZo_dCF-L0hontG_P+5yw@mail.gmail.com>
Date:   Fri, 9 Mar 2018 11:54:04 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     James Morris <jmorris@...ei.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        Paul Moore <paul@...l-moore.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Subject: Re: [PATCH v2] exec: Set file unwritable before LSM check

On Fri, Mar 9, 2018 at 11:47 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 9, 2018 at 11:30 AM, Kees Cook <keescook@...omium.org> wrote:
>> The LSM check should happen after the file has been confirmed to be
>> unchanging. Without this, we could have a race between the Time of Check
>> (the call to security_kernel_read_file() which could read the file and
>> make access policy decisions) and the Time of Use (starting with
>> kernel_read_file()'s reading of the file contents). In theory, file
>> contents could change between the two.
>
> I'm going to assume I get this for 4.17 from the security tree.
>
> Because I'm guessing there are actually no existing users that care?
> selinux seems to just look at file state, not actually at contents or
> anything that write access denial would care about.
>
> And the only other security module that even registers this is
> loadpin, and again it just seems to check things like "on the right
> filesystem" that aren't actually impacted by write access (in fact,
> the documented reason is to check that it's a read-only filesystem so
> that write access is simply _irrelevant_).
>
> So this issue seems to be mainly a cleanliness thing, not an actual bug.

That is my assumption too (I left off the Cc: stable as a result). I'm
much less familiar with IMA, though, but it's a caller of
kernel_read_file(), not hooking it, etc.

-Kees

-- 
Kees Cook
Pixel Security