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List:       freebsd-security
Subject:    Re: Cryptographic signatures of installer sets
From:       Glen Barber <gjb () freebsd ! org>
Date:       2020-01-30 13:22:39
Message-ID: 20200130132239.GG9584 () FreeBSD ! org
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Hi Nathan,

On Thu, Jan 30, 2020 at 12:50:06AM +0000, Nathan Dorfman wrote:
> On Mon, Jan 27, 2020 at 04:42:01PM +0000, Glen Barber wrote:
> > No, this last part is not true.  The installer always verifies the
> > checksums against /usr/freebsd-dist/MANIFEST on the installation medium.
> > 
> > In particular, this was done in r293223, where the LOCAL_DISTRIBUTIONS
> > variable explicitly contains the MANIFEST.
> 
> Thank you, Glen. You're correct of course; the installer uses its
> embedded MANIFEST and doesn't even fetch it from the mirror... during
> system installation, at least.
> 
> However, the first time a jail is set up, using the `bsdinstall jail`
> command, it does in fact fetch and trust the mirror's MANIFEST. I just
> tested this with a freshly installed 12.1-RELEASE system and a local
> mirror with a modified base.txz and manifest. It installs the modified
> files into the new jail without any complaint.
> 
> Simply, after a clean installation /usr/freebsd-dist doesn't exist on
> the new system, so the jail script creates it and downloads the MANIFEST
> from the mirror. See lines 60-70, here:
> 
> https://svnweb.freebsd.org/base/release/12.1.0/usr.sbin/bsdinstall/scripts/jail?view=markup#l60
> 
> After the first jail, this downloaded manifest and package(s) are saved
> in /usr/freebsd-dist. So you are only at risk the first time, and there
> will be some evidence of the tampering. Still, I hope you'll agree that
> this should be fixed. The installer already has a trusted manifest as
> you point out, why not simply install that one into the target system's
> /usr/freebsd-dist at setup time?
> 

I honestly wasn't aware there was a jail subcommand to bsdinstall.
I think, rather than creating /usr/freebsd-dist on the host system, we
should instead check if the misc/freebsd-release-manifests package is
installed and bail if it does not.  This package contains the MANIFEST
files from past releases (and in-progress releases, including BETA and
RC builds).

Does that seem like a reasonable solution?

Glen


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