Re: [PATCH v12 13/26] userns: Add pointer to ima_namespace to user_namespace

Christian Brauner
2022-05-23T12:42:15Z

On Mon, May 23, 2022 at 07:31:29AM -0400, Stefan Berger wrote:
>
>
> On 5/23/22 05:59, Christian Brauner wrote:
> > On Sun, May 22, 2022 at 01:24:26PM -0500, Serge Hallyn wrote:
> > > On Wed, Apr 20, 2022 at 10:06:20AM -0400, Stefan Berger wrote:
> > > > Add a pointer to ima_namespace to the user_namespace and initialize
> > > > the init_user_ns with a pointer to init_ima_ns. We need a pointer from
> > > > the user namespace to its associated IMA namespace since IMA namespaces
> > > > are piggybacking on user namespaces.
> > > >
> > > > Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
> > > > Acked-by: Christian Brauner <brauner@kernel.org>
> > > > Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
> > > >
> > > > ---
> > > > v11:
> > > > - Added lost A-b from Christian back
> > > > - Added sentence to patch description explaining why we need the pointer
> > > >
> > > > v9:
> > > > - Deferred implementation of ima_ns_from_user_ns() to later patch
> > > > ---
> > > > include/linux/ima.h | 2 ++
> > > > include/linux/user_namespace.h | 4 ++++
> > > > kernel/user.c | 4 ++++
> > > > 3 files changed, 10 insertions(+)
> > > >
> > > > diff --git a/include/linux/ima.h b/include/linux/ima.h
> > > > index 426b1744215e..fcb60a44e05f 100644
> > > > --- a/include/linux/ima.h
> > > > +++ b/include/linux/ima.h
> > > > @@ -14,6 +14,8 @@
> > > > #include <crypto/hash_info.h>
> > > > struct linux_binprm;
> > > > +extern struct ima_namespace init_ima_ns;
> > > > +
> > > > #ifdef CONFIG_IMA
> > > > extern enum hash_algo ima_get_current_hash_algo(void);
> > > > extern int ima_bprm_check(struct linux_binprm *bprm);
> > > > diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> > > > index 33a4240e6a6f..019e8cf7b633 100644
> > > > --- a/include/linux/user_namespace.h
> > > > +++ b/include/linux/user_namespace.h
> > > > @@ -36,6 +36,7 @@ struct uid_gid_map { /* 64 bytes -- 1 cache line */
> > > > #define USERNS_INIT_FLAGS USERNS_SETGROUPS_ALLOWED
> > > > struct ucounts;
> > > > +struct ima_namespace;
> > > > enum ucount_type {
> > > > UCOUNT_USER_NAMESPACES,
> > > > @@ -99,6 +100,9 @@ struct user_namespace {
> > > > #endif
> > > > struct ucounts *ucounts;
> > > > long ucount_max[UCOUNT_COUNTS];
> > > > +#ifdef CONFIG_IMA_NS
> > >
> > > It's probably worth putting a comment here saying that user_ns does not
> > > pin ima_ns.
> > >
> > > That the only time the ima_ns will be freed is when user_ns is freed,
> > > and only time it will be changed is when user_ns is freed, or during
> > > ima_fs_ns_init() (under smp_load_acquire) during a new mount.
> > >
> > > > + struct ima_namespace *ima_ns;
> > >
> > > So, if I create a new user_ns with a new ima_ns, and in there I
> > > create a new user_ns again, it looks like ima_ns will be NULL in
> > > the new user_ns? Should it not be set to the parent->ima_ns?
> > > (which would cause trouble for the way it's currently being
> > > freed...)
> >
> > Would also work and wouldn't be difficult to do imho.
>
> We previously decide to create an ima_namespace when securityfs is mounted.
> This now also applies to nested containers where an IMA namespace is first
> configured with the hash and template to use in a particular container and
> then activated. If no configuration is done it will inherit the hash and
> template from the first ancestor that has been configure when it is
> activated. So the same steps and behavior applies to *all* containers, no
> difference at any depth of nesting. Besides that, we don't want nested
> containers to share policy and logs but keep them isolated from each other,
> or do we not?
>
> Further, how should it work if we were to apply this even to the first
> container? Should it just inherit the &init_ima_namespace and we'd have no
> isolation at all? Why would we start treating containers at deeper nesting
> levels differently?

Valid points. I understood Serge as suggesting an implementation detail
change not a design change but might misunderstand him here.

# Currently

1. create new userns -> imans set to NULL
2. mount securityfs and configure imans -> set imans to &new_ima_ns

When 2. hasn't been done then we find the relevant imans by walking
the userns hierarchy upwards finding the first parent userns that has a
non-NULL imans.

# Serge's suggestion

1. create new userns -> imans is set to parent imans
2. mount securityfs and configure imans -> replace parent with &new_ima_ns

So when 2. hasn't been done we don't need to walk the userns hierarchy
upwards. We always find the relevant imans directly. Some massaging
would be needed in process_measurement() probably but it wouldn't need
to change semantics per se.

But I think I misunderstood something in any case. So looking at an
example like ima_post_path_mknod(). You seem to not call into
ima_must_appraise() if the caller's userns doesn't have an imans
enabled. I somehow had thought that the same logic applied as in
process_measurement. But if it isn't then it might make sense to keep
the current implementation.