linux-kernel - [09/55] virtio: Decrement avail idx on buffer detach

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Message-Id: <20110429185653.460564522@clark.kroah.org>
Date:	Fri, 29 Apr 2011 11:55:32 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Amit Shah <amit.shah@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: [09/55] virtio: Decrement avail idx on buffer detach

2.6.38-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Amit Shah <amit.shah@...hat.com>

commit b3258ff1d6086bd2b9eeb556844a868ad7d49bc8 upstream.

When detaching a buffer from a vq, the avail.idx value should be
decremented as well.

This was noticed by hot-unplugging a virtio console port and then
plugging in a new one on the same number (re-using the vqs which were
just 'disowned').  qemu reported

   'Guest moved used index from 0 to 256'

when any IO was attempted on the new port.

Reported-by: juzhang <juzhang@...hat.com>
Signed-off-by: Amit Shah <amit.shah@...hat.com>
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 drivers/virtio/virtio_ring.c |    1 +
 1 file changed, 1 insertion(+)

--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -371,6 +371,7 @@ void *virtqueue_detach_unused_buf(struct
 		/* detach_buf clears data, so grab it now. */
 		buf = vq->data[i];
 		detach_buf(vq, i);
+		vq->vring.avail->idx--;
 		END_USE(vq);
 		return buf;
 	}


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