Return-path: Received: from mail-la0-f46.google.com ([209.85.215.46]:33592 "EHLO mail-la0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750832AbbETB3o (ORCPT ); Tue, 19 May 2015 21:29:44 -0400 Received: by lagr1 with SMTP id r1so51025101lag.0 for ; Tue, 19 May 2015 18:29:42 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <1432084019.4510.242.camel@linux.vnet.ibm.com> References: <20150519200232.GM23057@wotan.suse.de> <1432072117.4510.180.camel@linux.vnet.ibm.com> <20150519221902.GQ23057@wotan.suse.de> <1432078625.4510.207.camel@linux.vnet.ibm.com> <1432084019.4510.242.camel@linux.vnet.ibm.com> From: Andy Lutomirski Date: Tue, 19 May 2015 18:29:22 -0700 Message-ID: (sfid-20150520_033000_879195_E68469AF) Subject: Re: [RFD] linux-firmware key arrangement for firmware signing To: Mimi Zohar Cc: "Luis R. Rodriguez" , Matthew Garrett , Rusty Russell , Casey Schaufler , Andy Lutomirski , linux-security-module , 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 , Konstantin Ryabitsev , Kees Cook Content-Type: text/plain; charset=UTF-8 Sender: linux-wireless-owner@vger.kernel.org List-ID: On Tue, May 19, 2015 at 6:06 PM, Mimi Zohar wrote: > On Tue, 2015-05-19 at 17:22 -0700, Luis R. Rodriguez wrote: >> On Tue, May 19, 2015 at 4:37 PM, Mimi Zohar wrote: >> > On Wed, 2015-05-20 at 00:19 +0200, Luis R. Rodriguez wrote: >> >> On Tue, May 19, 2015 at 05:48:37PM -0400, Mimi Zohar wrote: >> >> > On Tue, 2015-05-19 at 22:02 +0200, Luis R. Rodriguez wrote: > >> > In this case, not only is there a >> > security hook, but the IMA hook exists as well. To appraise firmware, >> > add a line to the IMA policy containing "appraise func=FIRMWARE_CHECK". >> > Similarly, to add a measurement to the measurement list, add a line to >> > the IMA policy containing "measure func=FIRMWAE_CHECK". >> >> I have a series of reasons find IMA unsuitable for the current goals at hand: >> >> 1) IMA is a pretty big kitchen sink, we want this to work well for >> even embedded systems, or architectures that do not have or require >> TPMs > > There are different aspects to IMA. One aspect is collecting file > measurements and extending the TPM with those measurements. The other > aspect is appraising file integrity. For that aspect, IMA-appraisal > does not use a TPM. > >> 2) The appraisal is also done for to account for a specific state of >> affairs, you appraise to the user of the integrity of the system at a >> specific point in time, > > True, IMA can be used to attest to the integrity of a system. > >> firmware signing can provide integrity / >> authorship vetting of files directly from the authors. > > It can also be used to appraise the integrity of a file, be it an > executable, a kernel module, configuration file or firmware in a > consistent manor, based on a file hash or signature. > >> In the case of >> regulatory.bin that was the whole point of it, and firmware signing as >> is being provided is intended to generalize that but by sharing code >> in-kernel with module signing infrastructure > > The underlying code used to verify the file signatures is the same. > The difference being where/how the file signatures are stored and which > keys to trust. > >> I am in hopes some others might be able to chime in more on point 2) here. >> >> Don't get me wrong IMA is nice, but its a big chunky requirement to >> have, more than what module signing provides and what it requires >> today to replace subsystem file signing requirements. > >> Now, LSM hooks -- that's more aligned with something we can start IMHO >> reasonably arguing we should shift module signing code to be punted >> into. But I've heard stories of LSM having issues with some virtual >> environments, and LSM stacking is also pretty new, and IMHO that'd be >> one way to compartmentalize all this module signing code. IMHO that >> *should happen* but can only be taken seriously once LSM stacking is >> merged in and baked. Its not, but I'm excited for it. > > Have you even looked at IMA-appraisal? I just looked extremely briefly. It seems to be much simpler than the PKCS#7 thing. OTOH, it seems to hardcode some rather scary assumptions that it's using RSA in digsig_asymmetric.c, specifically: pks.rsa.s = mpi_read_raw_data(hdr->sig, siglen); This bit in ima_appraise_measurement ignores trailing junk. Why? if (xattr_len - sizeof(xattr_value->type) - hash_start >= iint->ima_hash->length) /* xattr length may be longer. md5 hash in previous version occupied 20 bytes in xattr, instead of 16 */ rc = memcmp(&xattr_value->digest[hash_start], iint->ima_hash->digest, iint->ima_hash->length); I got confused around here: /* Replace RSA with HMAC if not mounted readonly and * not immutable */ if (!IS_RDONLY(dentry->d_inode) && !IS_IMMUTABLE(dentry->d_inode)) evm_update_evmxattr(dentry, xattr_name, xattr_value, xattr_value_len); Huh? Anyway, AFAICT IMA is about tracking the integrity of an FS that's being actively modified, not about distributing signed things. Also, I couldn't spot what part of IMA detects an attacker replacing one signed file with a different one from the same filesystem but a different name. --Andy