Messages in this thread | | | Date | Sat, 21 Aug 2021 01:53:42 +0800 | Subject | Re: [PATCH] ima: fix infinite loop within "ima_match_policy" function. | From | liqiong <> |
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Hi Simon,
On 2021/8/20 21:23, 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. I got it, how about this: /* * The previous list has to traverse to new list, * Or there may be a deadly loop within * "list_for_each_entry_rcu(entry, ima_rules, list)" * * That is tricky, after updated "ima_rules", restore the previous list. */ > > >> + 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 :)
Sorry for the mistake, I copy/paste the patch and delete/edit some lines, have reviewed before sending, but not found. I have made a case to reproduce the error, dumping "ima_rules" and every item address of list in the error situaiton, I can watchthe "ima_rules" change, old list traversing to the new list. And I have been doing a reboot test which found this bug. This patch seems to work fine.
> >> + 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.
Yes, It's the workflow.
> 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?
Yes, "ima_rules" cache not update in time, It's a risk. I am not sure if "WRITE_ONCE" can do this trick. How about: WRITE_ONCE(prev_rules->next, policy->next); WRITE_ONCE(ima_rules, policy);
If can't fix the cache issue, maybe the "ima_rules_tmp" solution is the best way. I will test it.
> 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
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