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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 23 Dec 2016 11:42:45 -0800
Message-ID: <CA+55aFxA1LE+4jdR8YXZyRVJ-MpOCUVob=ESBsMPuW=Qb=px-A@mail.gmail.com>
Subject: Re: [4.10, panic, regression] iscsi: null pointer deref at iscsi_tcp_segment_done+0x20d/0x2e0
To: Christoph Hellwig <hch@lst.de>
Cc: Chris Leech <cleech@redhat.com>, Ming Lei <tom.leiming@gmail.com>,
        Dave Chinner <david@fromorbit.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Lee Duncan <lduncan@suse.com>, open-iscsi@googlegroups.com,
        Linux SCSI List <linux-scsi@vger.kernel.org>,
        linux-block <linux-block@vger.kernel.org>, Jens Axboe <axboe@fb.com>,
        "Michael S. Tsirkin" <mst@redhat.com>
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On Fri, Dec 23, 2016 at 2:00 AM, Christoph Hellwig <hch@lst.de> wrote:
>
> From: Christoph Hellwig <hch@lst.de>
> Date: Fri, 23 Dec 2016 10:57:06 +0100
> Subject: virtio_blk: avoid DMA to stack for the sense buffer
>
> Most users of BLOCK_PC requests allocate the sense buffer on the stack,
> so to avoid DMA to the stack copy them to a field in the heap allocated
> virtblk_req structure.  Without that any attempt at SCSI passthrough I/O,
> including the SG_IO ioctl from userspace will crash the kernel.  Note that
> this includes running tools like hdparm even when the host does not have
> SCSI passthrough enabled.

Ugh. This patch is nasty.

I think we should just fix blk_execute_rq() instead.

But from a quick look, we also have at least sg_scsi_ioctl() and
sg_io() doing the same thing.

And the SG_IO thing in bsg_ioctl(). And spi_execute() in scsi_transport_spi.c

And resp_requests() in scsi_debug.c.

So I guess ugly it may need to be, and the rule is that the sense
buffer really can be on the stack and you can't DMA to/from it.
Comments from others?

                Linus