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Re: [dm-devel] [BUG] 2.6.39.1 crash in scsi_dispatch_cmd()



On Wed, Jul 06, 2011 at 01:06:44AM -0700, Roland Dreier wrote:
>  > > I booted with slub_debug=FZUP and the 6b pattern in RAX pretty much
>  > > does prove that this is a use-after-free issue.  Any thoughts about
>  > > how to pin this down before I muddle on in my lowbrow way?
>  >
>  > Alan Stern came up with a patch that could fix this:
>  >
>  > http://marc.info/?l=linux-kernel&m=130963676907731&w=2
> 
> Thanks!  It seems my crash is actually a different problem (or perhaps
> the same problem in different code), since I'm not going through SCSI
> directly but rather a dm-multipath device on top of SCSI disks.  But
> it does seem that it is another case of the block queue elevator
> getting freed while requests can still be submitted.
> 
> In my case, to reproduce this I have to hold the multipath device file
> open with something like "cat > /dev/dm-X" and then kill the
> underlying drive.  What I think is happening (although I haven't
> traced all the layers to be sure) is that then the multipath daemon
> notices that all the paths to the disk are lost and tries to kill the
> multipath device, which ends up in dm.c:dm_destroy().
> 
> This ends up in blk_cleanup_queue() which frees q->elevator, which of
> course leads to the crash.

So it's the identical crash that I reported also (well, at least one of
the crashes I reported).

> What's not clear to me is how things are supposed to work.  It seems
> that the dm stuff at least is missing a lot of required reference
> counting, to make sure that some structure sticks around to reject IOs
> to the device after it is destroy but while it is still open.  But I
> don't understand why people don't hit this more since it is completely
> reproducible for me with a fairly normal setup (hot-remove a multipath
> device that some process has open).

cc'ing Alasdair as well, maybe he knows...

> Alan Stern's patch looks a bit fishy -- the scsi_free_queue() is moved
> earlier than the
> 
> 	/* cause the request function to reject all I/O requests */
> 	sdev->request_queue->queuedata = NULL;
> 
> which seems to leave a small window where the use-after-free can
> happen, and it's not clear to me why the scsi_free_queue() has to move
> at all.
> 
> Thanks,
>  - R.
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