Messages in this thread | | | Date | Wed, 30 Nov 2022 16:16:39 +0100 | From | Michal Koutný <> | Subject | Re: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg |
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On Tue, Nov 29, 2022 at 03:34:00PM -0500, Waiman Long <longman@redhat.com> wrote: > The reproducing system can no longer produce a warning with this patch. > All the runnable block/0* tests including block/027 were run successfully > without failure.
Thanks for the test!
> @@ -1088,7 +1088,15 @@ static void blkcg_destroy_blkgs(struct blkcg *blkcg) > > might_sleep(); > > - css_get(&blkcg->css); > + /* > + * blkcg_destroy_blkgs() shouldn't be called with all the blkcg > + * references gone and rcu_read_lock not held. > + */ > + if (!css_tryget(&blkcg->css)) { > + WARN_ON_ONCE(!rcu_read_lock_held()); > + return; > + }
As I followed the previous discussion, the principle is that obtaining a reference or being inside an RCU read section is sufficient.
Consequently, I'd expect the two situations handled equally but here the no-ref but RCU bails out. (Which is OK because blkg_list must be empty?)
However, the might_sleep() in (non-sleepable) RCU reader section combo makes me wary anyway (not with the early return but tools would likely complain).
All in all, can't the contract of blkcg_destroy_blkgs() declare that a caller must pass blkcg with a valid reference? (The body of blkcg_destroy_blkgs then wouldn't need to get neither put the inner reference).
HTH, Michal [unhandled content-type:application/pgp-signature] | |