linux-kernel - Re: [RFC PATCH v9 02/16] ipe: add policy parser

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Message-ID: <CAHC9VhTkEHvoSFu8h=tuGJAjPhohj7ABPi+XXVg4j5MesCbtxw@mail.gmail.com>
Date:   Tue, 11 Apr 2023 15:13:26 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Fan Wu <wufan@...ux.microsoft.com>
Cc:     corbet@....net, zohar@...ux.ibm.com, jmorris@...ei.org,
        serge@...lyn.com, tytso@....edu, ebiggers@...nel.org,
        axboe@...nel.dk, agk@...hat.com, snitzer@...nel.org,
        eparis@...hat.com, linux-doc@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
        dm-devel@...hat.com, linux-audit@...hat.com,
        roberto.sassu@...wei.com, linux-kernel@...r.kernel.org,
        Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [RFC PATCH v9 02/16] ipe: add policy parser

On Thu, Apr 6, 2023 at 4:00 PM Fan Wu <wufan@...ux.microsoft.com> wrote:
> On Thu, Mar 02, 2023 at 02:02:32PM -0500, Paul Moore wrote:
> > On Mon, Jan 30, 2023 at 5:58???PM Fan Wu <wufan@...ux.microsoft.com> wrote:
> > >
> > > From: Deven Bowers <deven.desai@...ux.microsoft.com>
> > >
> > > IPE's interpretation of the what the user trusts is accomplished through
> > > its policy. IPE's design is to not provide support for a single trust
> > > provider, but to support multiple providers to enable the end-user to
> > > choose the best one to seek their needs.
> > >
> > > This requires the policy to be rather flexible and modular so that
> > > integrity providers, like fs-verity, dm-verity, dm-integrity, or
> > > some other system, can plug into the policy with minimal code changes.
> > >
> > > Signed-off-by: Deven Bowers <deven.desai@...ux.microsoft.com>
> > > Signed-off-by: Fan Wu <wufan@...ux.microsoft.com>
> >
> > ...
> >
> > > ---
> > >  security/ipe/Makefile        |   2 +
> > >  security/ipe/policy.c        |  99 +++++++
> > >  security/ipe/policy.h        |  77 ++++++
> > >  security/ipe/policy_parser.c | 515 +++++++++++++++++++++++++++++++++++
> > >  security/ipe/policy_parser.h |  11 +
> > >  5 files changed, 704 insertions(+)
> > >  create mode 100644 security/ipe/policy.c
> > >  create mode 100644 security/ipe/policy.h
> > >  create mode 100644 security/ipe/policy_parser.c
> > >  create mode 100644 security/ipe/policy_parser.h

...

> > > diff --git a/security/ipe/policy_parser.c b/security/ipe/policy_parser.c
> > > new file mode 100644
> > > index 000000000000..c7ba0e865366
> > > --- /dev/null
> > > +++ b/security/ipe/policy_parser.c
> > > @@ -0,0 +1,515 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * Copyright (C) Microsoft Corporation. All rights reserved.
> > > + */
> > > +
> > > +#include "policy.h"
> > > +#include "policy_parser.h"
> > > +#include "digest.h"
> > > +
> > > +#include <linux/parser.h>
> > > +
> > > +#define START_COMMENT  '#'
> > > +
> > > +/**
> > > + * new_parsed_policy - Allocate and initialize a parsed policy.
> > > + *
> > > + * Return:
> > > + * * !IS_ERR   - OK
> > > + * * -ENOMEM   - Out of memory
> > > + */
> > > +static struct ipe_parsed_policy *new_parsed_policy(void)
> > > +{
> > > +       size_t i = 0;
> > > +       struct ipe_parsed_policy *p = NULL;
> > > +       struct ipe_op_table *t = NULL;
> > > +
> > > +       p = kzalloc(sizeof(*p), GFP_KERNEL);
> > > +       if (!p)
> > > +               return ERR_PTR(-ENOMEM);
> > > +
> > > +       p->global_default_action = ipe_action_max;
> >
> > I'm assuming you're using the "ipe_action_max" as an intentional bogus
> > placeholder value here, yes?  If that is the case, have you considered
> > creating an "invalid" enum with an explicit zero value to save you
> > this additional assignment (you are already using kzalloc())?  For
> > example:
> >
> >   enum ipe_op_type {
> >     IPE_OP_INVALID = 0,
> >     IPE_OP_EXEC,
> >     ...
> >     IPE_OP_MAX,
> >   };
> >
> >   enum ipe_action_type {
> >     IPE_ACTION_INVALID = 0,
> >     IPE_ACTION_ALLOW,
> >     ...
> >     IPE_ACTION_MAX,
> >   };
> >
>
> Yes, IPE_ACTION_MAX is kind of the INVALID value we are using here.
>
> But I think we might be adding unnecessary complexity by using the
> IPE_OP_INVLIAD enum here. Currently, we are using IPE_OP_MAX to
> represent the number of operations we have, and we have allocated
> an IPE_OP_MAX-sized array to store linked lists that link all rules
> for each operation. If we were to add IPE_OP_INVLIAD to the enum
> definition, then IPE_OP_MAX-1 would become the number of operations,
> and we would need to change the index used to access the linked list
> array.

Gotcha.  Thanks for the explanation, that hadn't occurred to me while
I was reviewing the code.

Another option would be to create a macro to help reinforce that the
"max" value is being used as an "invalid" value, for example:

#define IPE_OP_INVALID IPE_OP_MAX

-- 
paul-moore.com