linux-kernel - Re: [PATCH v17 17/21] ipe: enable support for fs-verity as a trust provider

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Message-ID: <20240425042004.GE1401@sol.localdomain>
Date: Wed, 24 Apr 2024 21:20:04 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Fan Wu <wufan@...ux.microsoft.com>
Cc: corbet@....net, zohar@...ux.ibm.com, jmorris@...ei.org,
	serge@...lyn.com, tytso@....edu, axboe@...nel.dk, agk@...hat.com,
	snitzer@...nel.org, eparis@...hat.com, paul@...l-moore.com,
	linux-doc@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org, fsverity@...ts.linux.dev,
	linux-block@...r.kernel.org, dm-devel@...ts.linux.dev,
	audit@...r.kernel.org, linux-kernel@...r.kernel.org,
	Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [PATCH v17 17/21] ipe: enable support for fs-verity as a trust
 provider

On Wed, Apr 24, 2024 at 08:42:33PM -0700, Eric Biggers wrote:
> On Fri, Apr 12, 2024 at 05:56:00PM -0700, Fan Wu wrote:
> > +config IPE_PROP_FS_VERITY
> > +	bool "Enable property for fs-verity files"
> > +	depends on FS_VERITY && FS_VERITY_BUILTIN_SIGNATURES
> > +	help
> > +	  This option enables the usage of properties "fsverity_signature"
> > +	  and "fsverity_digest". These properties evaluate to TRUE when
> > +	  a file is fsverity enabled and has a valid builtin signature
> > +	  whose signing cert is in the .fs-verity keyring or its
> > +	  digest matches the supplied value in the policy.
> > +
> > +	  if unsure, answer Y.
> 
> Does this really need to depend on FS_VERITY_BUILTIN_SIGNATURES?  That's needed
> for fsverity_signature to work, but fsverity_digest would work without it.
> 
> I'd prefer if people had the option of only turning on
> FS_VERITY_BUILTIN_SIGNATURES if they really need it.
> 

I see that IPE_PROP_DM_VERITY is auto-selected when
DM_VERITY && DM_VERITY_VERIFY_ROOTHASH_SIG.  That differs from
IPE_PROP_FS_VERITY.  Should they really differ in this way?

Would it perhaps make more sense to not have the IPE_PROP_DM_VERITY and
IPE_PROP_FS_VERITY kconfig options at all, and instead just support the
corresponding IPE properties when the underlying kconfig options are enabled
(and SECURITY_IPE is also enabled)?

    DM_VERITY => dmverity_roothash
    DM_VERITY_VERIFY_ROOTHASH_SIG => dmverity_signature
    FS_VERITY => fsverity_digest
    FS_VERITY_BUILTIN_SIGNATURES => fsverity_signature

That would keep the number of kconfig options down, while also not forcing
people to enable the signature support in dm-verity and fsverity if they'd like
to use digests only.

- Eric