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

On 02/28/13 01:42, Jun'ichi Nomura wrote:
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.

Hello Jun'ichi,

Thanks for the feedback. It is good to know that there is no risk of triggering a use-after-free with the current approach.

How about reposting these patches as a performance optimization ? With these patches I see a slightly lower latency and slightly higher throughput. With a dm-linear mapping on top of a RAM disk (brd), a request size of 512 bytes and 100% reads fio reports 2063K IOPS without these patches and 2083K IOPS with these two patches applied. That's an improvement of about 1%. It's not much but that comes on top of the advantage that these two patches make the rq_completed() implementation easier to understand and to reason about.


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