Return-path: Received: from mail-la0-f54.google.com ([209.85.215.54]:35312 "EHLO mail-la0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751080AbbESXm1 (ORCPT ); Tue, 19 May 2015 19:42:27 -0400 Received: by labbd9 with SMTP id bd9so49224717lab.2 for ; Tue, 19 May 2015 16:42:25 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: References: <20150519200232.GM23057@wotan.suse.de> <555BA438.2070802@kernel.org> From: Andy Lutomirski Date: Tue, 19 May 2015 16:42:05 -0700 Message-ID: (sfid-20150520_014245_361538_B0FF0542) Subject: Re: [RFD] linux-firmware key arrangement for firmware signing To: Julian Calaby Cc: Andy Lutomirski , "Luis R. Rodriguez" , LSM List , James Morris , "Serge E. Hallyn" , "linux-kernel@vger.kernel.org" , linux-wireless , David Howells , Kyle McMartin , David Woodhouse , Seth Forshee , Greg Kroah-Hartman , Joey Lee , Rusty Russell , Mimi Zohar , Konstantin Ryabitsev , Michal Marek , Abelardo Ricart III , Sedat Dilek , keyrings@linux-nfs.org, Borislav Petkov , Jiri Kosina , Linus Torvalds Content-Type: text/plain; charset=UTF-8 Sender: linux-wireless-owner@vger.kernel.org List-ID: On Tue, May 19, 2015 at 4:30 PM, Julian Calaby wrote: > Hi All, > > On Wed, May 20, 2015 at 6:59 AM, Andy Lutomirski wrote: >> [added cc's from the other thread] >> >> On 05/19/2015 01:02 PM, Luis R. Rodriguez wrote: >>> >>> David Howells has posted v4 of his series of supporting PKCS#7 for module >>> signing. I'm in my v3 series now on RFCs for firmware PKCS#7 support, and >>> after >>> some review and patch shuffling I think this is ready for patch form. My >>> own >>> series however depend on quite a bit of other pending changes, one series >>> which >>> will go through Rusty's tree, another series of fixes on firmware_class >>> which >>> should go through Greg's tree. I'll wait until all this and David's own >>> patches >>> get merged before posting firmware PKCS#7 support. Before all this though >>> in >>> preparation for fw signing one thing we should start to talk about more >>> broadly >>> however is how linux-firmware binary file signing would work in practice >>> and >>> what we need, and make sure folks are OK with all this. >>> >>> First, firmware signing will be completely optional as with module >>> signing. >>> >> >> ... >> >>> Other than this last nitpick, any other concerns or recommendations ? >> >> >> A couple. Some of these are general concerns with the existing >> infrastructure, but #1 is a specific problem that gets much worse if we add >> firmware signing. Feel free to ignore 2-4. >> >> 1. We should get the signature semantics right. I think that, for modules, >> we currently sign literally the module payload. For modules, in my >> semi-amateurish crypto universe [1], this is fine *as long as the key in >> question is used for no other purpose*. For firmware, it's dangerous, since >> it would be vulnerable to substitution attacks in which the adversary >> convinces us to interpret one firmware file as firmware for another device >> or purpose entirely. >> >> We should be signing something that's semantically equivalent to "This is a >> valid module: xyz", "This is a valid 'regulatory.bin': xyz", or "This is a >> valid kexec image: xyz". > > Something that occurred to me (as a complete bystander) was: would it > make sense to have keys able to be restricted to particular "types" of > signable data? I.e. the key that can sign a valid regulatory.bin file > cannot be used to sign a module or a kexec image. - This could remove > the need to have multiple keyrings. (Also, UEFI keys unless otherwise > tagged could be restricted to only signing bootloaders or kernels) Seems sensible to me. FWIW, I'm starting to think that UEFI-based validation of kexec images should be totally separate. It uses a nasty PE format with a hideous PKCS #7 formatted signature. Maybe that should be a completely separate piece of code. > > Also, are multiple signatures a sensible thing? E.g. regulatory.bin > gets signed by Seth, then Kyle, then $DISTRO and any one key is enough > to validate it. That might further complicate matters. --Andy