linux-kernel - Re: [RFC PATCH] fw_lockdown: new micro LSM module to prevent loading unsigned firmware

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Date:   Fri, 10 Nov 2017 14:50:30 -0500
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>
Cc:     David Howells <dhowells@...hat.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Johannes Berg <johannes@...solutions.net>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH] fw_lockdown: new micro LSM module to prevent
 loading unsigned firmware

On Fri, 2017-11-10 at 20:35 +0100, Luis R. Rodriguez wrote:
> On Fri, Nov 10, 2017 at 12:58:23PM -0500, Mimi Zohar wrote:
> > Hi David,
> > 
> > If you are interested in preventing the loading of unsigned firmware,
> > the patch below is straight forward.  The patch has ONLY been tested
> > with IMA-appraisal enabled, and works as intended - allowing only
> > signed firmware to be loaded.
> 
> Very nice! This is the sort of thing that I mean by LSM'ifying fw access
> through a system policy.
> 
> We currently handle the LSM aspect for firmware through
> kernel_read_file_from_path() and so the kernel_read_file LSM hook, so why a new
> hook here?

kernel_read_file(), itself, is not an LSM hook, but calls two LSM
hooks named security_kernel_read_file(), prior to reading a file, and
security_kernel_post_read_file(), post reading a file.

In this case, we want to reject even reading the file if it isn't
signed, so we're using the security_kernel_read_file() LSM hook.

> 
> Where does this plug in?

This is a standalone, micro LSM that can be configured at build.  For
now I left it is an optional Kconfig parameter, but at some point, you
might want to consider making it required.

Mimi