Re: [PATCH] Fix a use-after-free triggered by device removal

[Bart Van Assche <bvanassche@acm.org>]

On 09/06/12 20:14, Mike Christie wrote:
> On 09/06/2012 12:58 PM, Bart Van Assche wrote:
>> _suOn 09/06/12 18:27, Michael Christie wrote:
>>> On Sep 3, 2012, at 9:12 AM, Bart Van Assche <bvanassche@acm.org> wrote:
>>>> If the put_device() call in scsi_request_fn() drops the sdev refcount
>>>> to zero then the spin_lock() call after the put_device() call triggers
>>>> a use-after-free. Avoid that by making sure that blk_cleanup_queue()
>>>> can only finish after all active scsi_request_fn() calls have returned.
>>>
>>> If we have this patch http://marc.info/?l=linux-scsi&m=134453905402413&w=2
>>> it seems we have all the scsi layer callers of the request_fn/
>>> *blk_run_queue holding a reference to the device when they make the call.
>>> Right, or are there some other places missing?
>>>
>>> What are the other places we can call the request_fn without already
>>> holding a reference to the device? Is it the block layer? Is that why we
>>> need this patch?
>>
>> The purpose of this patch is indeed to make *blk_run_queue() calls from
>> the block layer safe. There are several direct or indirect
>> *blk_run_queue() calls in the block layer where a reference on the queue
>> is held but not on the sdev, e.g. in the md, dm and bsg drivers.
> 
> Is there a race still? If some blk code is calling blk_run_queue
> (waiting on the queue lock) but no IO is queued,
> blk_drain_queue/blk_cleanup_queue could complete since the drain
> counters are zero. Then blk_run_queue could grab the queue lock and call
> the request_fn on a freed scsi_device (sdev pointed to by q->queuedata
> would be freed so scsi_reuqest_fn would be freed).
> 
> Do we need a check in __blk_run_queue for QUEUE_FLAG_DEAD (if dead then
> fail IO?), or do we need a check in scsi_request_fn for this. A dead
> queue check or maybe null q->queuedata in
> scsi_device_dev_release_usercontext (do this with the queue lock held),
> then check for null q->queuedata in scsi_request_fn?

Yet another scenario is that scsi_remove_host() gets invoked and
finishes after scsi_request_fn() has unlocked the queue and before it
locks the queue again. That's a scenario that can't be handled by adding
more checks at the start of __blk_run_queue() or scsi_request_fn().

Bart.