Cryptographic signatures of installer sets

[Glen Barber]

On Sat, Jan 25, 2020 at 08:00:07PM +0000, Nathan Dorfman wrote:
> Hello all,
> 
> I really hope I'm missing something here, and we can all have a nice
> chuckle at my expense.
> 
> But I can't see any way the integrity of the installer sets (base.txz,
> kernel.txz and friends) can be verified cryptographically? There is a
> MANIFEST file containing SHA256 checksums, but it itself does not appear
> to be signed in any way.
> 
> The installer images do come with PGP-signed checksums. So, when using
> an image that already contains all the sets, one can be sure they are
> authentic. What happens when one uses a network-only installer, though?
> How can it authenticate the sets it downloads from the user's chosen
> mirror?
> 
> A cursory glance at src/usr.sbin/bsdinstall suggests that it does not,
> in fact, do that. Checksums are compared against the MANIFEST (in
> scripts/checksum), but that is itself simply downloaded from the same
> mirror (in scripts/jail), usually over plain FTP, without any
> authentication.
> 

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.

Glen

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