Return-path: Received: from e23smtp07.au.ibm.com ([202.81.31.140]:33623 "EHLO e23smtp07.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751241AbbETCGy (ORCPT ); Tue, 19 May 2015 22:06:54 -0400 Received: from /spool/local by e23smtp07.au.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 20 May 2015 12:06:51 +1000 Message-ID: <1432087551.4510.260.camel@linux.vnet.ibm.com> (sfid-20150520_040718_028346_4D5D62A6) Subject: Re: [RFD] linux-firmware key arrangement for firmware signing From: Mimi Zohar To: Andy Lutomirski 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 Date: Tue, 19 May 2015 22:05:51 -0400 In-Reply-To: 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> Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Sender: linux-wireless-owner@vger.kernel.org List-ID: On Tue, 2015-05-19 at 18:29 -0700, Andy Lutomirski wrote: > 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? The same field is used to store either an MD5 or a SHA1 hash. The hash length is dependent on the hash algorithm > 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? EVM signatures are replaced with a keyed HMAC. > > Anyway, AFAICT IMA is about tracking the integrity of an FS that's > being actively modified, not about distributing signed things. Files that are signed are considered immutable and do not change (ima_appraise_measurement: case EVM_IMA_XATTR_DIGSIG). Fin posted RPM patches to store file signatures in the RPM header. Similarly, I posted patches to include file signatures in deb packages. (We still need to upstream these patches.) > 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. Perhaps you're referring to EVM? Mimi