[PATCH] ima: fix infinite loop within "ima_match_policy" function.

[李力琼]

Hi Mimi :

The situation is a little different,'list_splice_init_rcu'
don't change the list head. If "ima_rules" being changed,
readers may can't reload the new value in time for cpu cache
or compiler optimization. Defining "ima_rules" as a volatile 
variable can fix, but It is inefficient.

Maybe using a temporary ima_rules variable for every 
"list_for_each_entry_rcu(entry, ima_rules, list)" loop is 
a better solution to fix the "endless loop" bug. 

Regards,

liqiong

在 2021年08月20日 23:48, Mimi Zohar 写道:
> On Fri, 2021-08-20 at 13:23 +0000, THOBY Simon wrote:
>> Hi Liqiong,
>>
>> On 8/20/21 12:15 PM, 李力琼 wrote:
>>> Hi, Simon:
>>>
>>> This solution is better then rwsem, a temp "ima_rules" variable should 
>>> can fix. I also have a another idea, with a little trick, default list
>>> can traverse to the new list, so we don't need care about the read side. 
>>>
>>> here is the patch:
>>>
>>> @@ -918,8 +918,21 @@ void ima_update_policy(void)
>>>         list_splice_tail_init_rcu(&ima_temp_rules, policy, synchronize_rcu);
>>>
>>>         if (ima_rules != policy) {
>>> +               struct list_head *prev_rules = ima_rules;
>>> +               struct list_head *first = ima_rules->next;
>>>                 ima_policy_flag = 0;
>>> +
>>> +               /*
>>> +                * Make the previous list can traverse to new list,
>>> +                * that is tricky, or there is a deadly loop whithin
>>> +                * "list_for_each_entry_rcu(entry, ima_rules, list)"
>>> +                *
>>> +                * After update "ima_rules", restore the previous list.
>>> +                */
>> I think this could be rephrased to be a tad clearer, I am not quite sure
>> how I must interpret the first sentence of the comment.
>>
>>
>>> +               prev_rules->next = policy->next;
>>>                 ima_rules = policy;
>>> +               syncchronize_rcu();
>> I'm a bit puzzled as you seem to imply in the mail this patch was tested,
>> but there is no 'syncchronize_rcu' (with two 'c') symbol in the kernel.
>> Was that a copy/paste error? Or maybe you forgot the 'not' in "This
>> patch has been tested"? These errors happen, and I am myself quite an
>> expert in doing them :)
>>
>>> +               prev_rules->next = first;
>>>
>>>
>>> The side effect is the "ima_default_rules" will be changed a little while.
>>> But it make sense, the process should be checked again by the new policy.
>>>
>>> This patch has been tested, if will do, I can resubmit this patch.> 
>>> How about this ?
>> least
>>
>> Correct me if I'm wrong, here is how I think I understand you patch.
>> We start with a situation like that (step 0):
>> ima_rules --> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0
>>
>> Then we decide to update the policy for the first time, so
>> 'ima_rules [&ima_default_rules] != policy [&ima_policy_rules]'.
>> We enter the condition.
>> First we copy the current value of ima_rules (&ima_default_rules)
>> to a temporary variable 'prev_rules'. We also create a pointer dubbed
>> 'first' to the entry 1 in the default list (step 1):
>> prev_rules -------------
>>                        \/
>> ima_rules --> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0
>>                                                                    /\
>> first --------------------------------------------------------------
>>
>>
>> Then we update prev_rules->next to point to policy->next (step 2):
>> List entry 1 <-> List entry 2 <-> ... -> List entry 0
>>  /\
>> first
>> 	(notice that list entry 0 no longer points backwards to 'list entry 1',
>> 	but I don't think there is any reverse iteration in IMA, so it should be
>> 	safe)
>>
>> prev_rules -------------
>>                        \/
>> ima_rules --> List entry 0 (head node) = ima_default_rules   
>>                        |
>>                        |
>>                        -------------------------------------------
>>                                                                  \/
>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0'
>>
>>
>> We then update ima_rules to point to ima_policy_rules (step 3):
>> List entry 1 <-> List entry 2 <-> ... -> List entry 0
>>  /\
>> first
>>
>> prev_rules -------------
>>                        \/
>> ima_rules     List entry 0 (head node) = ima_default_rules   
>>      |                 |
>>      |                 |
>>      |                 ------------------------------------------
>>      ---------------                                            |
>>                    \/                                           \/
>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0'
>>                                                   synchronize_rcu                 /\
>> first --------------------------------------------------------------
>>
>> Then we run synchronize_rcu() to wait for any RCU reader to exit their loops (step 4).
>>
>> Finally we update prev_rules->next to point back to the ima policy and fix the loop (step 5):
>>
>> List entry 1 <-> List entry 2 <-> ... -> List entry 0
>>  /\
>> first
>>
>> prev_rules ---> List entry 0 (head node) = ima_default_rules <-> List entry 1 <-> List entry 2 <-> ... <-> List entry 0
>>                                                                      /\
>>                                                                  first (now useless)
>> ima_rules        
>>      |
>>      |
>>      |
>>      ---------------
>>                    \/
>> policy --> policy entry 0' (head node) = ima_policy_rules <-> policy entry 1' <-> policy entry 2' <-> .... <-> policy entry 0'
>>
>> The goal is that readers should still be able to loop
>> (forward, as we saw that backward looping is temporarily broken)
>> while in steps 0-4.
>>
>> I'm not completely sure what would happen to a client that started iterating
>> over ima_rules right after step 2.
>>
>> Wouldn't they be able to start looping through the new policy
>> as 'List entry 0 (head node) = ima_default_rules' points to ima_policy_rules?
>> And if they, wouldn't they loop until the write to 'ima_rule' at step 3 (admittedly
>> very shortly thereafter) completed?
>> And would the compiler be allowed to optimize the read to 'ima_rules' in the
>> list_for_each_entry() loop, thereby never reloading the new value for
>> 'ima_rules', and thus looping forever, just what we are trying to avoid?
>>
>> Overall, I'm tempted to say this is perhaps a bit too complex (at least,
>> my head tells me it is, but that may very well be because I'm terrible
>> at concurrency issues).
>>
>> Honestly, in this case I think awaiting input from more experienced
>> kernel devs than I is the best path forward :-)
> I'm far from an expert on RCU locking, but __list_splice_init_rcu()
> provides an example of how to make sure there aren't any readers
> traversing the list, before two lists are spliced together.   In our
> case, after there aren't any readers, instead of splicing two lists
> together, it should be safe to point to the new list.
>
> thanks,
>
> Mimi
>