Re: [RFC 2/2] initramfs with digital signature protection

From: Mimi Zohar
Date: Fri Apr 12 2013 - 07:54:41 EST

On Thu, 2013-04-11 at 10:52 -0400, Vivek Goyal wrote:
> On Wed, Apr 10, 2013 at 05:05:22PM -0400, Mimi Zohar wrote:
> > On Wed, 2013-04-10 at 15:42 -0400, Vivek Goyal wrote:
> > > On Tue, Apr 09, 2013 at 11:07:10PM -0400, Mimi Zohar wrote:
> > >
> > > [..]
> > > > The module keyring is a special case. Loading these keys from the
> > > > kernel and, presumably, locking the keyring is probably fine. In the
> > > > case of IMA, however, files will be signed by any number of package
> > > > owners. If the _ima keyring is locked by the kernel, how would you add
> > > > these other keys?
> > >
> > > Who are package owners here. IOW, in typical IMA setup, where are the keys
> > > and when are these keys loaded in ima keyring?
> >
> > Suppose I install third party packages not signed by the distro, but by
> > the package owner (eg. google, rpmfusion, ...). Not only does the
> > package signature need to be verified on installation, but the files
> > need to be installed with signatures. For IMA to enforce file
> > integrity, the package owner's public key needs to be added to the _ima
> > keyring.
> Ok, got it.
> >
> > > If we trust root and keys can be loaded any time later, then signed
> > > initramfs will not solve the problem either.
> >
> > Locking the keyring in the kernel will limit the set of permitted keys
> > to only those specified in UEFI db or builtin. Locking the keyring in
> > the "early" initramfs, will allow the system owner, whose key is in the
> > UEFI db, to specify additional keys, such as those for third party
> > packages. Not all public keys belong in the UEFI db.
> Ok, so third party public key you don't want to add in UEFI db. Instead
> platform owner (whose key is in UEFI db) will regenerate initrafs and
> sign it.
> So above use case is primarily for user space files and verifying its
> integrity and IMA keyring is used for that. secureboot does not require
> IMA keyring to be locked as none of that code runs at ring0.
> This locking requirement of IMA keyring is coming in only because we are
> trying to also verify the integrity of a process who will load code which
> runs at ring0.
> So how do you like the idea of using another keyring (say system keyring)
> for this purpose and what keyring to use integrity of a file can be
> encoded in file signature.
> This is something similar to INTEGRITY_KEYRING_MODULE keyring for modules.
> (Though I don't see this code fully hooked up yet).
> Or, given the fact that module signature and here /sbin/kexec verification
> will make use of similar keys, we can create a system keyring, make module
> code make use of that keyring. And IMA code can make use of that keyring
> too if a file's digital signature indicates so.

Sounds good to me!



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