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

[Stefan Berger <stefanb@linux.ibm.com>]

On 5/23/22 10:25, Serge E. Hallyn wrote:
> On Mon, May 23, 2022 at 02:41:59PM +0200, Christian Brauner wrote:
>> 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.
> 
> 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.

> 
>> # 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.