Messages in this thread | | | Date | Wed, 30 Nov 2022 10:44:36 -0500 | Subject | Re: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg | From | Waiman Long <> |
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On 11/30/22 10:16, Michal Koutný wrote: > 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).
You are right. I should have pushed the might_sleep down(). Will post a new version to fix that.
Thanks, Longman
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