linux-kernel - Re: [PATCH] SELinux: Measure state and hash of policy using IMA

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Message-ID: <f041e8ee-3955-9551-b72d-d4d7fa6e636d@linux.microsoft.com>
Date:   Mon, 24 Aug 2020 14:29:13 -0700
From:   Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>
To:     Ondrej Mosnacek <omosnace@...hat.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>
Cc:     Mimi Zohar <zohar@...ux.ibm.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        tusharsu@...ux.microsoft.com, Sasha Levin <sashal@...nel.org>,
        James Morris <jmorris@...ei.org>,
        linux-integrity@...r.kernel.org,
        SElinux list <selinux@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] SELinux: Measure state and hash of policy using IMA

On 8/24/20 1:01 PM, Ondrej Mosnacek wrote:
> On Mon, Aug 24, 2020 at 9:30 PM Stephen Smalley
> <stephen.smalley.work@...il.com> wrote:
>> On Mon, Aug 24, 2020 at 2:13 PM Lakshmi Ramasubramanian
>> <nramas@...ux.microsoft.com> wrote:
>>>
>>> On 8/24/20 7:00 AM, Stephen Smalley wrote:
>>>
>>>>> +int security_read_policy_kernel(struct selinux_state *state,
>>>>> +                               void **data, size_t *len)
>>>>> +{
>>>>> +       int rc;
>>>>> +
>>>>> +       rc = security_read_policy_len(state, len);
>>>>> +       if (rc)
>>>>> +               return rc;
>>>>> +
>>>>> +       *data = vmalloc(*len);
>>>>> +       if (!*data)
>>>>> +               return -ENOMEM;
>>>>>
>>>>> +       return security_read_selinux_policy(state, data, len);
>>>>>    }
>>>>
>>>> See the discussion here:
>>>> https://lore.kernel.org/selinux/20200824113015.1375857-1-omosnace@redhat.com/T/#t
>>>>
>>>> In order for this to be safe, you need to ensure that all callers of
>>>> security_read_policy_kernel() have taken fsi->mutex in selinuxfs and
>>>> any use of security_read_policy_len() occurs while holding the mutex.
>>>> Otherwise, the length can change between security_read_policy_len()
>>>> and security_read_selinux_policy() if policy is reloaded.
>>>>
>>>
>>> "struct selinux_fs_info" is available when calling
>>> security_read_policy_kernel() - currently in measure.c.
>>> Only "struct selinux_state" is.
>>>
>>> Is Ondrej's re-try approach I need to use to workaround policy reload issue?
>>
>> No, I think perhaps we should move the mutex to selinux_state instead
>> of selinux_fs_info.  selinux_fs_info has a pointer to selinux_state so
>> it can then use it indirectly.  Note that your patches are going to
>> conflict with other ongoing work in the selinux next branch that is
>> refactoring policy load and converting the policy rwlock to RCU.
> 
> Yeah, and I'm experimenting with a patch on top of Stephen's RCU work
> that would allow you to do this in a straightforward way without even
> messing with the fsi->mutex. My patch may or may not be eventually
> committed, but either way I'd recommend holding off on this for a
> while until the dust settles around the RCU conversion.
> 

I can make the SELinux\IMA changes in "selinux next branch" taking 
dependencies on Stephen's patches + relevant IMA patches.

Could you please let me know the URL to the "selinux next branch"?

thanks,
  -lakshmi