On 5/23/22 10:25, Serge E. Hallyn wrote:

On Mon, May 23, 2022 at 02:41:59PM +0200, Christian Brauner wrote:

quoted

On Mon, May 23, 2022 at 07:31:29AM -0400, Stefan Berger wrote:

quoted

On 5/23/22 05:59, Christian Brauner wrote:

quoted

On Sun, May 22, 2022 at 01:24:26PM -0500, Serge Hallyn wrote:

quoted

On Wed, Apr 20, 2022 at 10:06:20AM -0400, Stefan Berger wrote:

quoted

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.

quoted

+	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.

Ah, right, thanks Christian.

But so the code - I think where the ima_ns is defined in the user_ns
struct, needs to make this clear.  Just something like

	// Pointer to ima_ns which this user_ns created.  Can be null.
	// Access checks will walk the userns->parent chain and check
	// against all active ima_ns's.  Note that when the user_ns is
	// freed, the ima_ns is guaranteed to be free-able.
	struct ima_namespace	*ima_ns;

I added this comment now. Thanks.

quoted

# 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.