Re: [PATCH-block v2] bdi, blk-cgroup: Fix potential UAF of blkcg

["Michal Koutný" <mkoutny@suse.com>]

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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

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