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

[THOBY Simon]

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 ?


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'
                                                                   /\
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 :-)

> 
> ----------
> Regards,
> liqiong
> 

Thanks,
Simon