Re: [PATCH block/for-linus] block: blkg_destroy_all() should clear q->root_blkg and ->root_rl.blkg

["Richard W.M. Jones" <rjones@redhat.com>]

On Sat, Sep 05, 2015 at 03:47:36PM -0400, Tejun Heo wrote:
> While making the root blkg unconditional, ec13b1d6f0a0 ("blkcg: always
> create the blkcg_gq for the root blkcg") removed the part which clears
> q->root_blkg and ->root_rl.blkg during q exit.  This leaves the two
> pointers dangling after blkg_destroy_all().  blk-throttle exit path
> performs blkg traversals and dereferences ->root_blkg and can lead to
> the following oops.
> 
>  BUG: unable to handle kernel NULL pointer dereference at 0000000000000558
>  IP: [<ffffffff81389746>] __blkg_lookup+0x26/0x70
>  ...
>  task: ffff88001b4e2580 ti: ffff88001ac0c000 task.ti: ffff88001ac0c000
>  RIP: 0010:[<ffffffff81389746>]  [<ffffffff81389746>] __blkg_lookup+0x26/0x70
>  ...
>  Call Trace:
>   [<ffffffff8138d14a>] blk_throtl_drain+0x5a/0x110
>   [<ffffffff8138a108>] blkcg_drain_queue+0x18/0x20
>   [<ffffffff81369a70>] __blk_drain_queue+0xc0/0x170
>   [<ffffffff8136a101>] blk_queue_bypass_start+0x61/0x80
>   [<ffffffff81388c59>] blkcg_deactivate_policy+0x39/0x100
>   [<ffffffff8138d328>] blk_throtl_exit+0x38/0x50
>   [<ffffffff8138a14e>] blkcg_exit_queue+0x3e/0x50
>   [<ffffffff8137016e>] blk_release_queue+0x1e/0xc0
>  ...
> 
> While the bug is a straigh-forward use-after-free bug, it is tricky to
> reproduce because blkg release is RCU protected and the rest of exit
> path usually finishes before RCU grace period.
> 
> This patch fixes the bug by updating blkg_destro_all() to clear
> q->root_blkg and ->root_rl.blkg.
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: "Richard W.M. Jones" <rjones@redhat.com>
> Reported-by: Josh Boyer <jwboyer@fedoraproject.org>
> Link: http://lkml.kernel.org/g/CA+5PVA5rzQ0s4723n5rHBcxQa9t0cW8BPPBekr_9aMRoWt2aYg@mail.gmail.com
> Fixes: ec13b1d6f0a0 ("blkcg: always create the blkcg_gq for the root blkcg")
> Cc: stable@vger.kernel.org # v4.2+
> ---
> Hello,
> 
> Richard, can you please verify that this patch fixes the bug?

This patch managed 477 iterations before dying from an unrelated
reason in the test harness.  This is much better than before, so the
patch looks good to me.

Rich.

> Thanks a lot!
> 
>  block/blk-cgroup.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index d6283b3..9cc48d1d 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -387,6 +387,9 @@ static void blkg_destroy_all(struct request_queue *q)
>  		blkg_destroy(blkg);
>  		spin_unlock(&blkcg->lock);
>  	}
> +
> +	q->root_blkg = NULL;
> +	q->root_rl.blkg = NULL;
>  }
>  
>  /*

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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