pukkamustard schreef op wo 02-02-2022 om 11:10 [+0000]:
> The ERIS URN is only used if the entire narinfo is signed with a
> authorized signature.

Perhaps I'm missing something here, but in that case, shouldn't "ERIS"
be added to %mandatory-fields in (guix narinfo)?

Anyway, I don't see what prevents an unauthorised narinfo with a ERIS
URN to be used: the narinfo is chosen with

  (define narinfo
    (lookup-narinfo cache-urls store-item
                    (if (%allow-unauthenticated-substitutes?)
                        (const #t)
                        (cut valid-narinfo? <> acl))))

where lookup-narinfo is a tiny wrapper around lookup-narinfos/diverse.
lookup-narinfos/diverse considers both unauthorised and authorised
narinfos, and can choose an unauthorised narinfo if it's ‘equivalent’
to an authorised narinfo (using equivalent-narinfo?)

equivalent-narinfo? only looks at the hash, path, references and size,
and ignores the ERIS.  As such, an unauthorised narinfo with a
malicious ERIS URN could be selected.

However, it turns out that all this doesn't really matter: whether the
port returned by 'fetch' in (guix scripts substitute) came from
file://, http://, https:// or ERIS, the file hash is verified later
anyway:

                  ;; Compute the actual nar hash as we read it.
                  ((algorithm expected)
                   (narinfo-hash-algorithm+value narinfo))
                  ((hashed get-hash)
                   (open-hash-input-port algorithm input)))

      [...]

      ;; Check whether we got the data announced in NARINFO.
      (let ((actual (get-hash)))
        (if (bytevector=? actual expected)
            [...]

False alarm I guess!

Greetings,
Maxime.