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

From: Stefan Berger <stefanb@linux.ibm.com>
To: Christian Brauner <brauner@kernel.org>,
	"Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-integrity@vger.kernel.org, zohar@linux.ibm.com,
	christian.brauner@ubuntu.com, containers@lists.linux.dev,
	dmitry.kasatkin@gmail.com, ebiederm@xmission.com,
	krzysztof.struczynski@huawei.com, roberto.sassu@huawei.com,
	mpeters@redhat.com, lhinds@redhat.com, lsturman@redhat.com,
	puiterwi@redhat.com, jejb@linux.ibm.com, jamjoom@us.ibm.com,
	linux-kernel@vger.kernel.org, paul@paul-moore.com,
	rgb@redhat.com, linux-security-module@vger.kernel.org,
	jmorris@namei.org, jpenumak@redhat.com
Subject: Re: [PATCH v12 13/26] userns: Add pointer to ima_namespace to user_namespace
Date: Mon, 23 May 2022 07:31:29 -0400	[thread overview]
Message-ID: <e1df20d5-a6c9-d30c-3671-46f7a8742bc0@linux.ibm.com> (raw)
In-Reply-To: <20220523095932.adr2r26o2obch4r5@wittgenstein>

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?