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Re: [dm-devel] [PATCH v2 0/2] dm: Avoid use-after-free of a mapped device

Hi Bart,

On 02/27/13 23:45, Bart Van Assche wrote:
> This mini-series of two patches avoids that the device mapper
> implementation can trigger a use-after-free during removal of a
> mapped device. The two patches in this series are:
> - block: Convert blk_run_queue() recursion into iteration.
> - dm: Avoid running the md queue after the last dm_put().
> Note: these patches are the result of source reading. As far as I know this issue has not (yet) caused any harm.

Ref-counting of mapped device is like this:
  - dm depends on the fact that the block device is opened while there
    is bio/request submitted.  So dm_get/put in dm_blk_open/close is
    enough to keep mapped device while there are bios.
  - Request-based target has a tiny window between dm_blk_close()
    and the end of rq_completed() because the opener may close the device
    once the last bio completes even if request is still finishing.
    dm_get/dm_put in dm_start_request/rq_completed closes this window.
    (See comments in dm_start_request())
  - So, when dm_put() puts the last reference, there should be no
    requests in the queue.
  - If there is no reference to the mapped device, dm_destroy() may
    start tearing it down.
    It is ok if there is pending delayed work for the request queue
    because blk_cleanup_queue() is called before freeing the mapped device
    and cancels the delayed work.

So as far as blk_run_queue_async() in rq_completed() is concerned,
it is not a problem from "use-after-free" point of view.

Jun'ichi Nomura, NEC Corporation

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