linux-kernel - Re: 2.6.36-rc3: EIP is at scsi_init

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Date:	Thu, 09 Sep 2010 14:51:13 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	James Bottomley <James.Bottomley@...e.de>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.36-rc3: EIP is at scsi_init_io+...

On 2010-09-09 14:47, James Bottomley wrote:
> On Wed, 2010-09-08 at 17:12 -0700, Linus Torvalds wrote:
>> Hmm. No noise about this one.
>>
>> Jens, Fujita, James, any comments?
>>
>> On Mon, Aug 30, 2010 at 11:46 AM, Alexey Dobriyan <adobriyan@...il.com> wrote:
>>> Not much of a calltrace, it scrolled away because of hardlockup detector.
>>> On the bright side, radeon KMS worked correctly and actually showed it.
>>>
>>> $ addr2line -e vmlinux ffffffff812d207b
>>> drivers/scsi/scsi_lib.c:1015
>>>
>>>  1012  err_exit:
>>>  1013          scsi_release_buffers(cmd);
>>>  1014          scsi_put_command(cmd);
>>>  1015  ===>    cmd->request->special = NULL; <===
>>>  1016          return error;
>>>  1017  }
>>>  1018  EXPORT_SYMBOL(scsi_init_io);
>>
>> I do have to say that it looks rather wrong that it accesses "cmd"
>> after it has done the "scsi_put_command(cmd)" on it.
>>
>> I also note that that was introduced pretty recently by commit
>> 610a63498f7 ("scsi: fix discard page leak"), merged during this merge
>> window. That does look suspicious to me.
> 
> It's a use after free:  The put actually frees the cmnd and then we use
> it to get to the request.  Most of the time nothing notices, but if you
> have poison on free enabled, we may see the problem.  The fix is just to
> reverse the put and the set.

You are right, I misspoke in my original reply. It's clearing the
request field, not the command field (which would be bogus of course).

-- 
Jens Axboe

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