Re: [RFC] IMA LSM based rule race condition issue on 4.19 LTS

Guozihua (Scott)
2022-12-20T01:11:23Z

On 2022/12/19 21:11, Mimi Zohar wrote:
> On Mon, 2022-12-19 at 15:10 +0800, Guozihua (Scott) wrote:
>> On 2022/12/16 11:04, Paul Moore wrote:
>>> On Thu, Dec 15, 2022 at 9:36 PM Guozihua (Scott) <guozihua@huawei.com> wrote:
>>>> On 2022/12/16 5:04, Paul Moore wrote:
>>>
>>> ...
>>>
>>>>> How bad is the backport really? Perhaps it is worth doing it to see
>>>>> what it looks like?
>>>>>
>>>> It might not be that bad, I'll try to post a version next Monday.
>>>
>>> Thanks for giving it a shot.
>>>
>> When I am trying a partial backport of b16942455193 ("ima: use the lsm
>> policy update notifier"), I took a closer look into it and if we rip off
>> the RCU and the notifier part, there would be a potential UAF issue when
>> multiple processes are calling ima_lsm_update_rule() and
>> ima_match_rules() at the same time. ima_lsm_update_rule() would free the
>> old rule if the new rule is successfully copied and initialized, leading
>> to ima_match_rules() accessing a freed rule.
>>
>> To reserve the mainline solution, we would have to either introduce RCU
>> for rule access, which would work better with notifier mechanism or the
>> same rule would be updated multiple times, or we would have to introduce
>> a lock for LSM based rule update.
>
> Even with the RCU changes, the rules will be updated multiple times.
> With your "ima: Handle -ESTALE returned by ima_filter_rule_match()"
> patch, upstream makes a single local copy of the rule to avoid updating
> it multiple times. Without the notifier, it's updating all the rules.
That's true. However, in the mainline solution, we are only making a
local copy of the rule. In 4.19, because of the lazy update mechanism,
we are replacing the rule on the rule list multiple times and is trying
to free the original rule.
>
> Perhaps an atomic variable to detect if the rules are already being
> updated would suffice. If the atomic variable is set, make a single
> local copy of the rule.
That should do it. I'll send a patch set soon.

--
Best
GUO Zihua