Re: [RFC PATCH 0/2] ima: uncompressed module appraisal support

[Eric Snowberg]

> On Feb 12, 2020, at 7:04 AM, Nayna <nayna@xxxxxxxxxxxxxxxxxx> wrote:
> 
> 
> On 2/11/20 12:33 PM, Eric Snowberg wrote:
>>> On Feb 10, 2020, at 1:33 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>>> 
>>> On Mon, 2020-02-10 at 12:24 -0700, Eric Snowberg wrote:
>>>>> On Feb 10, 2020, at 10:09 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>>>>>> Ok, understood, âmodsigâ refers to strictly kernel module appended signatures
>>>>>> without regard to the keyring that verifies it.  Since there are inconsistencies
>>>>>> here, would you consider something like my first patch?  It will verify an
>>>>>> uncompressed kernel module containing an appended signature  when the public key
>>>>>> is contained within the kernel keyring instead of the ima keyring.  Why force a
>>>>>> person to add the same keys into the ima keyring for validation?  Especially when
>>>>>> the kernel keyring is now used to verify appended signatures in the compressed
>>>>>> modules.
>>>>> Different use case scenarios have different requirements.  Suppose for
>>>>> example that the group creating the kernel image is not the same as
>>>>> using it.  The group using the kernel image could sign all files,
>>>>> including kernel modules (imasig), with their own private key. Only
>>>>> files that they signed would be permitted.  Your proposal would break
>>>>> the current expectations, allowing kernel modules signed by someone
>>>>> else to be loaded.
>>>>> 
>>>> All the end user needs to do is compress any module created by the group that built
>>>> the original kernel image to work around the scenario above.  Then the appended
>>>> signature in the compressed module will be verified by the kernel keyring. Does
>>>> this mean there is a security problem that should be fixed, if this is a concern?
>>> Again, the issue isn't compressed/uncompressed kernel modules, but the
>>> syscall used to load the kernel module.  IMA can prevent using the the
>>> init_module syscall.  Refer to the ima_load_data() LOADING_MODULE
>>> case.
>> Within the ima_load_data() LOADING_MODULE case, to prevent IMA from using
>> the init_module syscall, is_module_sig_enforced() must return false. Currently
>> when is_module_sig_enforced() returns true, the kernel keyring is always used
>> for verification.
>> 
>> What if I change this part of my patch from
>> 
>> +       if (rc && func == MODULE_CHECK)
>> 
>> to
>> 
>> +       sig_enforce = is_module_sig_enforced();
>> +       if (sig_enforce && rc && func == MODULE_CHECK)
>> 
>> Now when the init_module syscall is available, finit_module syscall will use
>> both the ima keyring and kernel keyring for verification.  When the
>> init_module syscall is blocked from use, the finit_module syscall will only use
>> the ima keyring for validation.  I believe this would satisfy both your use
>> case and mine.
>> 
> There are two syscalls - init_module, finit_module - and two signature verification methods. The problem you are trying to address is the finit_module syscall, using both signature verification methods. Why enable both signature verification methods ?

I am enabling both in my patch since a person can turn around and use the other syscall by 
simply compressing their module.  Now their module is verified by a different keyring. 
Other than completely disabling the init_module syscall, which we donât do, there is nothing 
preventing them from doing that.  We have one kernel config per architecture. We build
and sign the modules with an appended signature.

I can not predict all the ways someone will use a kernel built from this single config.  
I do believe if someone has IMA working with module verification and appended signatures,
some are not going to understand why their module that was compressed and loading 
(via syscall init_module) suddenly fails to load (via syscall finit_module) once they 
uncompress it.