Re: [PATCH v3 1/9] LSM: Identify modules by more than name

From: Paul Moore <paul@paul-moore.com>
To: "Mickaël Salaün" <mic@digikod.net>
Cc: Casey Schaufler <casey@schaufler-ca.com>,
	casey.schaufler@intel.com, linux-security-module@vger.kernel.org,
	jmorris@namei.org, keescook@chromium.org,
	john.johansen@canonical.com, penguin-kernel@i-love.sakura.ne.jp,
	stephen.smalley.work@gmail.com, linux-kernel@vger.kernel.org,
	linux-api@vger.kernel.org
Subject: Re: [PATCH v3 1/9] LSM: Identify modules by more than name
Date: Sun, 27 Nov 2022 22:52:52 -0500	[thread overview]
Message-ID: <CAHC9VhSza-P0hG_iSdW8MCAKaykUW5eLBkpg=bb4-D_=7-j3+Q@mail.gmail.com> (raw)
In-Reply-To: <94ac3c49-550b-c517-680f-ba653d568f72@digikod.net>

On Fri, Nov 25, 2022 at 11:30 AM Mickaël Salaün <mic@digikod.net> wrote:
> On 23/11/2022 21:15, Casey Schaufler wrote:
> > Create a struct lsm_id to contain identifying information
> > about Linux Security Modules (LSMs). At inception this contains
> > the name of the module and an identifier associated with the
> > security module. Change the security_add_hooks() interface to
> > use this structure. Change the individual modules to maintain
> > their own struct lsm_id and pass it to security_add_hooks().
> >
> > The values are for LSM identifiers are defined in a new UAPI
> > header file linux/lsm.h. Each existing LSM has been updated to
> > include it's LSMID in the lsm_id.
> >
> > The LSM ID values are sequential, with the oldest module
> > LSM_ID_CAPABILITY being the lowest value and the existing modules
> > numbered in the order they were included in the main line kernel.
> > This is an arbitrary convention for assigning the values, but
> > none better presents itself. The value 0 is defined as being invalid.
> > The values 1-99 are reserved for any special case uses which may
> > arise in the future.
> >
> > Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> > ---
> >   include/linux/lsm_hooks.h    | 16 ++++++++++++++--
> >   include/uapi/linux/lsm.h     | 32 ++++++++++++++++++++++++++++++++
> >   security/apparmor/lsm.c      |  8 +++++++-
> >   security/bpf/hooks.c         | 13 ++++++++++++-
> >   security/commoncap.c         |  8 +++++++-
> >   security/landlock/cred.c     |  2 +-
> >   security/landlock/fs.c       |  2 +-
> >   security/landlock/ptrace.c   |  2 +-
> >   security/landlock/setup.c    |  6 ++++++
> >   security/landlock/setup.h    |  1 +
> >   security/loadpin/loadpin.c   |  9 ++++++++-
> >   security/lockdown/lockdown.c |  8 +++++++-
> >   security/safesetid/lsm.c     |  9 ++++++++-
> >   security/security.c          | 12 ++++++------
> >   security/selinux/hooks.c     |  9 ++++++++-
> >   security/smack/smack_lsm.c   |  8 +++++++-
> >   security/tomoyo/tomoyo.c     |  9 ++++++++-
> >   security/yama/yama_lsm.c     |  8 +++++++-
> >   18 files changed, 141 insertions(+), 21 deletions(-)
> >   create mode 100644 include/uapi/linux/lsm.h

...

> > diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
> > new file mode 100644
> > index 000000000000..47791c330cbf
> > --- /dev/null
> > +++ b/include/uapi/linux/lsm.h
> > @@ -0,0 +1,32 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +/*
> > + * Linux Security Modules (LSM) - User space API
> > + *
> > + * Copyright (C) 2022 Casey Schaufler <casey@schaufler-ca.com>
> > + * Copyright (C) 2022 Intel Corporation
> > + */
> > +
> > +#ifndef _UAPI_LINUX_LSM_H
> > +#define _UAPI_LINUX_LSM_H
> > +
> > +/*
> > + * ID values to identify security modules.
> > + * A system may use more than one security module.
> > + *
> > + * Values 1-99 are reserved for future use in special cases.
>
> This line should be removed unless justified. What could be special
> about IDs? The syscalls already have a "flags" argument, which is enough.
>
> > + */
> > +#define LSM_ID_INVALID               0
>
> Reserving 0 is good, but it doesn't deserve a dedicated declaration.
> LSM_ID_INVALID should be removed.
>
>
> > +#define LSM_ID_CAPABILITY    100
>
> This should be 1…

No.  Scratch that, make that an emphatic "No".

If you want to argue for a different reserved low-number range, e.g.
something with a nice power-of-2 limit, I'm okay with that, but as I
wrote earlier I feel strongly we need to have a low-number reserved
range for potential future uses.

-- 
paul-moore.com