linux-kernel - Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel lockdown

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Message-Id: <1510193857.4484.95.camel@linux.vnet.ibm.com>
Date:   Wed, 08 Nov 2017 21:17:37 -0500
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Blunck <jblunck@...radead.org>,
        Julia Lawall <julia.lawall@...6.fr>,
        David Howells <dhowells@...hat.com>,
        Marcus Meissner <meissner@...e.de>, Gary Lin <GLin@...e.com>,
        linux-security-module@...r.kernel.org, gnomes@...rguk.ukuu.org.uk,
        linux-efi <linux-efi@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Matthew Garrett <mjg59@...gle.com>
Subject: Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel
 lockdown

> > IMHO that should just fail then, ie, a "locked down" kernel should not want to
> > *pass* a firmware signature if such thing could not be done.
> > 
> > Its no different than trying to verify a signed module on a "locked down" for
> > which it has no signature.
> > 
> > But perhaps I'm not understanding the issue well, let me know.
> 
> My point is quite simple:
> my_deviceA_init() {
>         err = request_firmware(&fw, "deviceA"); <--- (a)
>         if (err)
>                 goto err_request;
> 
>         err = verify_firmware(fw);  <--- (b)
>         if (err)
>                 goto err_verify;
> 
>         load_fw_to_deviceA(fw);     <--- (c)
>         ...
> }
> 
> As legacy device drivers does not have (b), there is no chance to
> prevent loading a firmware at (c) for locked-down kernel.
> 
> If you allow me to bring in yet another function, say
> request_firmware_signable(), which should be used in place of (a)
> for all verification-aware drivers, that would be fine.

I really don't understand why you need a new function.  The
request_firmware() eventually calls kernel_read_file_from_path(),
which already calls the pre and post LSM hooks.

IMA-appraisal is already on these hooks verifying the requested
firmware's signature.  For systems with "lockdown" enabled, but
without IMA-appraisal enabled, define a small, builtin LSM that sits
on these LSM hooks and denies the unsigned firmware requests.

Mimi

> In this case, all the invocation of request_firmware() in legacy code
> could be forced to fail in locked-down kernel.
> 
> But I think that "signable" should be allowed to be combined with other
> features of request_firmware variants like _(no)wait or _direct.
> 
> -Takahiro AKASHI