linux-kernel - Re: [PATCH 2/3] qemu: Implement virtio-pstore device

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Message-ID: <20160720082108.GA13233@stefanha-x1.localdomain>
Date:	Wed, 20 Jul 2016 09:21:08 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Kr??m???? <rkrcmar@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Anthony Liguori <aliguori@...zon.com>,
	Anton Vorontsov <anton@...msg.org>,
	Colin Cross <ccross@...roid.com>,
	Kees Cook <keescook@...omium.org>,
	Tony Luck <tony.luck@...el.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Minchan Kim <minchan@...nel.org>, kvm@...r.kernel.org,
	qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 2/3] qemu: Implement virtio-pstore device

On Wed, Jul 20, 2016 at 12:48:39AM +0900, Namhyung Kim wrote:
> Hello,
> 
> On Mon, Jul 18, 2016 at 11:03:53AM +0100, Stefan Hajnoczi wrote:
> > On Mon, Jul 18, 2016 at 01:37:40PM +0900, Namhyung Kim wrote:
> > > +static void virtio_pstore_handle_io(VirtIODevice *vdev, VirtQueue *vq)
> > > +{
> > > +    VirtIOPstore *s = VIRTIO_PSTORE(vdev);
> > > +    VirtQueueElement *elem;
> > > +    struct virtio_pstore_hdr *hdr;
> > > +    ssize_t len;
> > > +
> > > +    for (;;) {
> > > +        elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> > > +        if (!elem) {
> > > +            return;
> > > +        }
> > > +
> > > +        hdr = elem->out_sg[0].iov_base;
> > > +        if (elem->out_sg[0].iov_len != sizeof(*hdr)) {
> > > +            error_report("invalid header size: %u",
> > > +                         (unsigned)elem->out_sg[0].iov_len);
> > > +            exit(1);
> > > +        }
> > 
> > Please use iov_to_buf() instead of directly accessing out_sg[].  Virtio
> > devices are not supposed to assume a particular iovec layout.  In other
> > words, virtio_pstore_hdr could be split across multiple out_sg[] iovecs.
> > 
> > You must also copy in data (similar to Linux syscall implementations) to
> > prevent the guest from modifying data while the command is processed.
> > Such race conditions could lead to security bugs.
> 
> By accessing elem->out_sg[0].iov_base directly, I abused it as an
> in-and-out buffer.  But it seems not allowed by the virtio spec, do I
> have to use separate buffers for request and response?

Yes, a virtqueue element has (host read-only) out buffers followed by
(host write-only) in buffers.

Stefan

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