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

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Date:   Wed, 15 Nov 2017 14:56:57 -0500
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Johannes Berg <johannes@...solutions.net>,
        Matthew Garrett <mjg59@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        "AKASHI, Takahiro" <takahiro.akashi@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jan Blunck <jblunck@...radead.org>,
        Julia Lawall <julia.lawall@...6.fr>,
        Marcus Meissner <meissner@...e.de>, Gary Lin <GLin@...e.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel
 lockdown

On Wed, 2017-11-15 at 18:52 +0100, Luis R. Rodriguez wrote:
> On Wed, Nov 15, 2017 at 06:49:57AM -0500, Mimi Zohar wrote:
> > On Tue, 2017-11-14 at 21:50 +0100, Luis R. Rodriguez wrote:
> > 
> > > Johannes made cfg80211 recently just use request_firmware() now via commit on
> > > linux-next 90a53e4432 ("cfg80211: implement regdb signature checking") [0] as
> > > he got tired of waiting firmware signing, but note he implemented a signature
> > > checking on its own so he open codes verify_pkcs7_signature() after the
> > > request_firmware() call. If we are happy to live with this, then so be it.
> > > 
> > > [0] https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=90a53e4432b12288316efaa5f308adafb8d304b0
> > 
> > Johannes was tired of waiting?  Commit 5a9196d "ima: add support for
> > measuring and appraising firmware" has been in the kernel since linux-
> > 3.17.
> > 
> > The original firmware hook for verifying firmware signatures were
> > replaced with the common LSM pre and post kernel_read_file() hooks
> > in linux-4.6.y.
> > 
> > Even if you wanted to support firmware signature verification without
> > IMA-appraisal, it should be using the LSM hooks.
> 
> request_firmware() uses kernel_read_file_from_path() underneath the hood,
> and so its used for both:
> 
> 	/lib/firmware/regulatory.db
> 	/lib/firmware/regulatory.db.p7s

The firmware signature validation should occur as part of
kernel_read_file_from_path(), not as a stand alone verification.

Why not extend kernel_read_file_from_path() to pass the detached signature?  Since the signature would only be used for the verification, there's no need to return the open file descriptor.

Or if you prefer, call kernel_read_file_from_path() for the detached signature first, and then call it again for the firmware with a pointer to the detached signature.

> 
> The later only if CONFIG_CFG80211_REQUIRE_SIGNED_REGDB, which defaults
> to y anyway.
> 
> What I meant was that net/wireless/reg.c now open codes firmware signature
> validation on its own rather than using a helper. IMA appraisal will still
> be used if enabled given kernel_read_file_from_path() is used.
> 
> The open coding of the firmware signature check is what I wanted to highlight.

How are the keys in the CFG80211_EXTRA_REGDB_KEYDIR verified?  The
call to key_create_or_update() with the KEY_ALLOC_BYPASS_RESTRICTION
option by-passes any requirement that the keys in this directory are
signed.  This by-passes the concept of extending the secure boot
signature chain of trust.  To safely validate the keys use the
restrict_link_by_builtin_trusted option.

Mimi