Re: [PATCH 10/10] LSM: Blob sharing support for S.A.R.A and LandLock

[Kees Cook <keescook@chromium.org>]

On Thu, Sep 13, 2018 at 6:16 AM, Paul Moore <paul@paul-moore.com> wrote:
> On Thu, Sep 13, 2018 at 12:19 AM Kees Cook <keescook@chromium.org> wrote:
>> On Tue, Sep 11, 2018 at 9:42 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
>> > Two proposed security modules require the ability to
>> > share security blobs with existing "major" security modules.
>> > These modules, S.A.R.A and LandLock, provide significantly
>> > different services than SELinux, Smack or AppArmor. Using
>> > either in conjunction with the existing modules is quite
>> > reasonable. S.A.R.A requires access to the cred blob, while
>> > LandLock uses the cred, file and inode blobs.
>> >
>> > The use of the cred, file and inode blobs has been
>> > abstracted in preceding patches in the series. This
>> > patch teaches the affected security modules how to access
>> > the part of the blob set aside for their use in the case
>> > where blobs are shared. The configuration option
>> > CONFIG_SECURITY_STACKING identifies systems where the
>> > blobs may be shared.
>> >
>> > The mechanism for selecting which security modules are
>> > active has been changed to allow non-conflicting "major"
>> > security modules to be used together. At this time the
>> > TOMOYO module can safely be used with any of the others.
>> > The two new modules would be non-conflicting as well.
>> >
>> > Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
>> > ---
>> >  Documentation/admin-guide/LSM/index.rst | 14 +++--
>> >  include/linux/lsm_hooks.h               |  2 +-
>> >  security/Kconfig                        | 81 +++++++++++++++++++++++++
>> >  security/apparmor/include/cred.h        |  8 +++
>> >  security/apparmor/include/file.h        |  9 ++-
>> >  security/apparmor/include/lib.h         |  4 ++
>> >  security/apparmor/lsm.c                 |  8 ++-
>> >  security/security.c                     | 30 ++++++++-
>> >  security/selinux/hooks.c                |  3 +-
>> >  security/selinux/include/objsec.h       | 18 +++++-
>> >  security/smack/smack.h                  | 19 +++++-
>> >  security/smack/smack_lsm.c              | 17 +++---
>> >  security/tomoyo/common.h                | 12 +++-
>> >  security/tomoyo/tomoyo.c                |  3 +-
>> >  14 files changed, 200 insertions(+), 28 deletions(-)
>
> ...
>
>> > diff --git a/security/Kconfig b/security/Kconfig
>> > index 22f7664c4977..ed48025ae9e0 100644
>> > --- a/security/Kconfig
>> > +++ b/security/Kconfig
>> > @@ -36,6 +36,28 @@ config SECURITY_WRITABLE_HOOKS
>> >         bool
>> >         default n
>> >
>> > +config SECURITY_STACKING
>> > +       bool "Security module stacking"
>> > +       depends on SECURITY
>> > +       help
>> > +         Allows multiple major security modules to be stacked.
>> > +         Modules are invoked in the order registered with a
>> > +         "bail on fail" policy, in which the infrastructure
>> > +         will stop processing once a denial is detected. Not
>> > +         all modules can be stacked. SELinux, Smack and AppArmor are
>> > +         known to be incompatible. User space components may
>> > +         have trouble identifying the security module providing
>> > +         data in some cases.
>> > +
>> > +         If you select this option you will have to select which
>> > +         of the stackable modules you wish to be active. The
>> > +         "Default security module" will be ignored. The boot line
>> > +         "security=" option can be used to specify that one of
>> > +         the modules identifed for stacking should be used instead
>> > +         of the entire stack.
>> > +
>> > +         If you are unsure how to answer this question, answer N.
>>
>> I don't see a good reason to make this a config. Why shouldn't this
>> always be enabled?
>
> I do.  From a user perspective it is sometimes difficult to determine
> the reason behind a failed operation; its is a DAC based denial, the
> LSM, or some other failure?  Stacking additional LSMs has the
> potential to make this worse.  The boot time configuration adds to the
> complexity.

Let me try to convince you otherwise. :) The reason I think there's no
need for this is because the only functional change here is how
_TOMOYO_ gets stacked. And in my proposal, we can convert TOMOYO to be
enabled/disabled like LoadPin. Given the configs I showed, stacking
TOMOYO with the other major LSMs becomes a config (and/or boottime)
option.

The changes for TOMOYO are still needed even _with_ SECURITY_STACKING,
and I argue that the other major LSMs remain the same. It's only
infrastructure that has changed. So, I think having SECURITY_STACKING
actually makes things more complex internally (all the ifdefs, weird
enable logic) and for distros ("what's this stacking option", etc?)

> I think we should leave this decision to the individual distros so
> that they can make their own decision on LSM stacking based on the
> savviness of their user base and the quality of their support
> infrastructure.

If we reach the "extreme" stacking case, then yes, I want to make sure
we've got something that makes sense. But this first step doesn't get
us there, so I'd prefer we avoid making it overly complex. I think
everything else in this series _reduces_ complexity, except for this
new config.

-Kees

-- 
Kees Cook
Pixel Security