Re: [PATCH v5 15/16] ima: Move dentries into ima_namespace

Stefan Berger
2021-12-10T16:40:34Z

On 12/10/21 10:48, Mimi Zohar wrote:
> On Fri, 2021-12-10 at 10:32 -0500, Stefan Berger wrote:
>> On 12/10/21 10:26, Mimi Zohar wrote:
>>> On Fri, 2021-12-10 at 09:26 -0500, James Bottomley wrote:
>>>> On Fri, 2021-12-10 at 09:17 -0500, Stefan Berger wrote:
>>>>> On 12/10/21 08:02, Mimi Zohar wrote:
>>>>>> On Fri, 2021-12-10 at 07:40 -0500, Stefan Berger wrote:
>>>>>>> On 12/10/21 07:09, Mimi Zohar wrote:
>>>>>>>> On Fri, 2021-12-10 at 12:49 +0100, Christian Brauner wrote:
>>>>>>>>>> There's still the problem that if you write the policy,
>>>>>>>>>> making the file disappear then unmount and remount
>>>>>>>>>> securityfs it will come back. My guess for fixing this is
>>>>>>>>>> that we only stash the policy file reference,
>>>>>>>>>> create it if NULL but then set the pointer to PTR_ERR(-
>>>>>>>>>> EINVAL) or something and refuse to create it for that
>>>>>>>>>> value.
>>>>>>>>> Some sort of indicator that gets stashed in struct ima_ns
>>>>>>>>> that the file does not get recreated on consecutive mounts.
>>>>>>>>> That shouldn't be hard to fix.
>>>>>>>> The policy file disappearing is for backwards compatibility,
>>>>>>>> prior to being able to extend the custom policy. For embedded
>>>>>>>> usecases, allowing the policy to be written exactly once might
>>>>>>>> makes sense. Do we really want/need to continue to support
>>>>>>>> removing the policy in namespaces?
>>>>>>> I don't have an answer but should the behavior for the same
>>>>>>> #define in this case be different for host and namespaces? Or
>>>>>>> should we just 'select IMA_WRITE_POLICY and IMA_READ_POLICY' when
>>>>>>> IMA_NS is selected?
>>>>>> The latter option sounds good. Being able to analyze the namespace
>>>>>> policy is really important.
>>>>> Ok, I will adjust the Kconfig for this then. This then warrants the
>>>>> question whether to move the dentry into the ima_namespace. The
>>>>> current code looks like this.
>>>>>
>>>>> #if !defined(CONFIG_IMA_WRITE_POLICY) &&
>>>>> !defined(CONFIG_IMA_READ_POLICY)
>>>>> securityfs_remove(ns->policy_dentry);
>>>>> ns->policy_dentry = NULL;
>>>>> ns->policy_dentry_removed = true;
>>>>> #elif defined(CONFIG_IMA_WRITE_POLICY)
>>>>>
>>>>> With IMA_NS selecting IMA_WRITE_POLICY and IMA_READ_POLICY the above
>>>>> wouldn't be necessary anymore but I find it 'cleaner' to still have
>>>>> the dentry isolated rather than it being a global static as it was
>>>>> before...
>>>> This is really, really why you don't want the semantics inside the
>>>> namespace to differ from those outside, because it creates confusion
>>>> for the people reading the code, especially with magically forced
>>>> config options like this. I'd strongly suggest you either keep the
>>>> semantic in the namespace or eliminate it entirely.
>>>>
>>>> If you really, really have to make the namespace behave differently,
>>>> then use global variables and put a big comment on that code saying it
>>>> can never be reached once CONFIG_IMA_NS is enabled.
>>> The problem seems to be with removing the securityfs policy file.
>>> Instead of removing it, just make it inacessible for the "if
>>> !defined(CONFIG_IMA_WRITE_POLICY) && !defined(CONFIG_IMA_READ_POLICY)"
>>> case.
>> So we would then leave it up to the one building the kernel to select
>> the proper compile time options (suggested ones being IMA_WRITE_POLICY
>> and IMA_READ_POLICY being enabled?) and behavior of host and IMA
>> namespace is then the same per those options? Removing the file didn't
>> seem the problem to me but more like whether the host should ever behave
>> differently from the namespace.
> You proposed "select IMA_WRITE_POLICY and IMA_READ_POLICY'" when IMA_NS
> is selected. At least IMA_READ_POLICY should be enabled for
> namespaces.
>
> In addition, if removing the securityfs file after a custom policy is
> loaded complicates namespacing, then don't remove it.

If we just leave the selection of the compile time options to the user
(suggested ones being IMA_WRITE_POLICY

and IMA_READ_POLICY being enabled) we don't need to make any changes.

Choices are for IMA_NS:
1) Leave compile-time options to the user and suggest IMA_WRITE_POLICY and IMA_READ_POLICY
2) Auto-select IMA_WRITE_POLICY and IMA_READ_POLICY for IMA_NS and still remove file for non-IMA_NS case and have the dentry in the ima_namespace
3) Auto-select IMA_WRITE_POLICY and IMA_READ_POLICY for IMA_NS and still remove file for non-IMA_NS case and use global dentry
4) Changing mode bits on the file/inode to avoid having the dentry

for v6 I just leave things as they are and we can then see what we need.

>
> thanks,
>
> Mimi
>