Re: [RFC][PATCH 08/12] KEYS: PGP-based public key signature verification

[Roberto Sassu]

On 12/10/2018 5:58 PM, David Howells wrote:
> Roberto Sassu <roberto.sassu@xxxxxxxxxx> wrote:
>
> > > You need to consider what it is that the patch trying to achieve.
> >
> > I understood that the purpose is to check PGP signatures with built-in
> > keys or keys provided by the user. Since using the session keyring
> > caused the issue I reported, I thought it was ok to use the user
> > keyring.
> >
> > Just a note: the original patches were relying on KEY_FLAG_TRUSTED to
> > determine if a key is trusted; now the trustworthiness depends on the
> > type of keyring passed to pgp_verify_sig(). I removed the additional key
> > search in the user (session) keyring to prevent that signature
> > verification is done with a key provided by the user even when the
> > caller of pgp_verify_sig() expects that a trusted key is used. The
> > search in the session keyring is done if the caller of pgp_verify_sig()
> > sets the keyring pointer to NULL.
>
> Thinking about these patches further, as you point out, the way that trust is
> computed has changed.  There is no no KEY_FLAG_TRUSTED; rather, a key is
> assumed to be trusted if it's on a trusted keyring.
>
> *Getting* it onto that trusted keyring is then the trick.  There are two ways:
>
>   (1) A key can be loaded from a trusted source during boot (say a compiled in
>       set of keys).
>
>   (2) A key can be added to that keyring later, provided that the key is
>       verified by a key already in the ring and the ring hasn't been closed.
>
> What do we need to check PGP signatures?  Blobs or keys as well?

For my use case, verifying blobs (RPM headers or 'Release' for Debian-
based distributions) would be sufficient. The keys can be added at
compile time.


> Why does the user keyring need to be a fallback?  (I know the session keyring
> used to be a fallback when I first did these, but things have changed since
> then).

Users can verify signatures with the pgp_test key type (patch 10/12).
During the first attempt, pgp_test tries to use built-in keys. If
verification fails, pgp_test tries again with keys in the user keyring.
But if verification succeeds during the second attempt, the kernel
prints a warning saying that an untrusted key was used.


> Should we have a separate built-in keyring for PGP keys?  Actually, I suspect
> we should probably mark keys in some way with what they're permitted to be
> used for.

I kept PGP keys in the main keyring, so that verify_pgp_signature() uses
the same convention of verify_pkcs7_signature() (keyring == NULL: use
primary keys, keyring == 1: use secondary keys).

In your patches there was a capability checking mechanism, but it was
removed with commit db6c43bd2132.

Roberto


> David

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