linux-kernel - Re: [PATCH v2] [SCSI] sg: Fix user memory corruption when SG

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Date:	Wed, 07 Aug 2013 12:31:30 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Roland Dreier <roland@...nel.org>
CC:	David Milburn <dmilburn@...hat.com>, Jens Axboe <axboe@...nel.dk>,
	James Bottomley <JBottomley@...allels.com>,
	Costa Sapuntzakis <costa@...estorage.com>,
	Jörn Engel <joern@...estorage.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	David Jeffery <djeffery@...hat.com>
Subject: Re: [PATCH v2] [SCSI] sg: Fix user memory corruption when SG_IO is
 interrupted by a signal

On 13-08-07 11:50 AM, Roland Dreier wrote:
> On Wed, Aug 7, 2013 at 7:38 AM, David Milburn <dmilburn@...hat.com> wrote:
>> I was able to succesfully test this patch overnight, I had been experimenting with the
>> sg driver setting the BIO_NULL_MAPPED flag in sg_rq_end_io_usercontext for a orphan process
>> which prevented the corruption, but your solution seems much better.
>
> Very cool, thanks for the testing.
>
> I actually looked at using BIO_NULL_MAPPED as well, but it seemed a
> bit too fragile to me -- it had the right effect of skipping
> __bio_copy_iov(), and skipping the __free_pages() stuff in there is OK
> because sg owns its pages rather than the bio layer, but all that
> seemed vulnerable to being broken by an unrelated change.
>
> Out of curiousity, were you already working on this bug?  Because if
> you had fixed it a few weeks earlier we might not have spent so long
> wondering WTF was stomping on the memory of one of our processes :)

Roland,
So what kind of signal was leading to your "stomping on the memory"?
Was it user generated or something like SIGIO, SIGPIPE or a RT signal?

To get around the SG_IO ioctl restart problem (for non idempotent
SCSI commands) could we replace a -ERESTARTSYS return value
with -EINTR ?

As I noted in a previous post, for robust user space code using the
SG_IO ioctl, masking signals during the IO may help.


And what about bsg? Is it any better or worse than sg in the case
of interrupted SG_IO ioctls? Apart from the interface (sg_io_hdr
v3 versus v4) it should be a drop in replacement for sg.

Doug Gilbert


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