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

Mimi Zohar
2022-12-19T13:12:17Z

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.

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.

--
thanks,

Mimi