[RFC 0/2] ima: evm: Add kernel cmdline options to disable IMA/EVM

[Song Liu]

> On Dec 17, 2024, at 3:16 PM, Paul Moore <paul at paul-moore.com> wrote:
> 
> On Tue, Dec 17, 2024 at 5:47 PM Song Liu <songliubraving at meta.com> wrote:
>> 
>> If we use lsm= to control ima and evm, we will need the following
>> changes in ordered_lsm_parse(). We still need supporting logic
>> in ima and evm side, so that ima and evm are only initialized
>> when they are in lsm=.
>> 
>> Does this sound the right way forward?
> 
> Have you tested it?  What happens?  There is value in going through
> the testing process, especially if you haven't played much with the
> LSM code.

Yes, I tested both the original patches and the "lsm=xx" version. 

> 
> I'd also want to see a comment line in both places explaining why it
> is necessary to mark the LSM as enabled prior to actually adding it to
> @ordered_lsms.  Something along the lines of only parsing the
> parameter once should be sufficient.

Please see below for the explanation. I will add different words in 
the actual comments so they make more sense as comments

> 
>> diff --git i/security/security.c w/security/security.c
>> index 09664e09fec9..00271be3b0c1 100644
>> --- i/security/security.c
>> +++ w/security/security.c
>> @@ -365,6 +365,9 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
>>                        if (strcmp(lsm->name, name) == 0) {
>>                                if (lsm->order == LSM_ORDER_MUTABLE)
>>                                        append_ordered_lsm(lsm, origin);
>> +                               else if (lsm->order == LSM_ORDER_LAST)
>> +                                       set_enabled(lsm, true);

We need a flag here, saying we want to enable the lsm. We cannot do 
append_ordered_lsm() yet, otherwise, it will not be "last". 

>> +
>>                                found = true;
>>                        }
>>                }
>> @@ -386,7 +389,7 @@ static void __init ordered_lsm_parse(const char *order, const char *origin)
>> 
>>        /* LSM_ORDER_LAST is always last. */
>>        for (lsm = __start_lsm_info; lsm < __end_lsm_info; lsm++) {
>> -               if (lsm->order == LSM_ORDER_LAST)
>> +               if (lsm->order == LSM_ORDER_LAST && is_enabled(lsm))
>>                        append_ordered_lsm(lsm, "   last");

Before this change, lsm with order==LSM_ORDER_LAST is always considered
enabled, which is a bug (if I understand you and Casey correctly). 
To fix this, we need a flag from above saying we actually want to enable 
it. 

I personally think it is fine to use set_enabled() to set the flag. 
But I don't have a strong preference, we can add a different flag. 

Does this make sense?

Thanks,
Song