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

[Eric Snowberg]

> On Feb 10, 2020, at 10:09 AM, Mimi Zohar <zohar at linux.ibm.com> wrote:
> 
> On Mon, 2020-02-10 at 09:34 -0700, Eric Snowberg wrote:
>>> On Feb 8, 2020, at 4:43 PM, Mimi Zohar <zohar at linux.ibm.com> wrote:
>>> 
>>> On Fri, 2020-02-07 at 14:38 -0700, Eric Snowberg wrote:
>>>>> On Feb 7, 2020, at 11:54 AM, Mimi Zohar <zohar at linux.ibm.com> wrote:
>>>>> 
>>>>> On Fri, 2020-02-07 at 11:45 -0700, Eric Snowberg wrote:
>>>>>> 
>>>>>>> On Feb 7, 2020, at 11:28 AM, Mimi Zohar <zohar at linux.ibm.com> wrote:
>>>>>>> 
>>>>>>> On Fri, 2020-02-07 at 10:49 -0700, Eric Snowberg wrote:
>>>>>>>> 
>>>>>>>>> On Feb 7, 2020, at 10:40 AM, Mimi Zohar <zohar at linux.ibm.com> wrote:
>>>>>>>>> 
>>>>>>>>>> $ insmod ./foo.ko
>>>>>>>>>> insmod: ERROR: could not insert module ./foo.ko: Permission denied
>>>>>>>>>> 
>>>>>>>>>> last entry from audit log:
>>>>>>>>>> type=INTEGRITY_DATA msg=audit(1581089373.076:83): pid=2874 uid=0
>>>>>>>>>> auid=0 ses=1 subj=unconfined_u:unconfined_r:unconfined_t:s0-
>>>>>>>>>> s0:c0.c1023 op=appraise_data cause=invalid-signature comm="insmod"
>>>>>>>>>> name="/root/keys/modules/foo.ko" dev="dm-0" ino=10918365
>>>>>>>>>> res=0^]UID="root" AUID=“root"
>>>>>>>>>> 
>>>>>>>>>> This is because modsig_verify() will be called from within
>>>>>>>>>> ima_appraise_measurement(), 
>>>>>>>>>> since try_modsig is true.  Then modsig_verify() will return
>>>>>>>>>> INTEGRITY_FAIL.
>>>>>>>>> 
>>>>>>>>> Why is it an "invalid signature"?  For that you need to look at the
>>>>>>>>> kernel messages.  Most likely it can't find the public key on the .ima
>>>>>>>>> keyring to verify the signature.
>>>>>>>> 
>>>>>>>> It is invalid because the module has not been ima signed. 
>>>>>>> 
>>>>>>> With the IMA policy rule "appraise func=MODULE_CHECK
>>>>>>> appraise_type=imasig|modsig", IMA first tries to verify the IMA
>>>>>>> signature stored as an xattr and on failure then attempts to verify
>>>>>>> the appended signatures.
>>>>>>> 
>>>>>>> The audit message above indicates that there was a signature, but the
>>>>>>> signature validation failed.
>>>>>>> 
>>>>>> 
>>>>>> I do have  CONFIG_IMA_APPRAISE_MODSIG enabled.  I believe the audit message above 
>>>>>> is coming from modsig_verify in security/integrity/ima/ima_appraise.c.
>>>>> 
>>>>> Right, and it's calling:
>>>>> 
>>>>> 	rc = integrity_modsig_verify(INTEGRITY_KEYRING_IMA, modsig);
>>>>> 
>>>>> It's failing because it is trying to find the public key on the .ima
>>>>> keyring.  Make sure that the public needed to validate the kernel
>>>>> module is on the IMA keyring (eg. keyctl show %keyring:.ima).
>>>>> 
>>>> 
>>>> I know that will validate the module properly, but that is not what I’m 
>>>> trying to solve here. I thought the point of adding “|modsig” to the
>>>> ima policy was to tell ima it can either validate against an ima keyring OR 
>>>> default back to the kernel keyring.  This is what happens with the compressed
>>>> module.  There isn’t anything in the ima keyring to validate the compressed
>>>> modules and it loads when I add “|modsig”.
>>> 
>>> "modsig" has nothing to do with keyrings.  The term "modsig" is
>>> juxtaposed to "imasig".  "modsig" refers to kernel module appended
>>> signature. 
>> 
>> Ok, understood, “modsig” 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?

For the use case above, wouldn’t it be better to use a module policy like:

appraise func=MODULE_CHECK appraise_type=imasig

Obviously the downside is it disables appended signatures. It would prevent 
compressed modules from loading, and only allow ima signed modules to load.