[PATCH 4.9 27/31] xen/blkfront: harden blkfront against event channel storms

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Juergen Gross <jgross@xxxxxxxx>

commit 0fd08a34e8e3b67ec9bd8287ac0facf8374b844a upstream.

The Xen blkfront driver is still vulnerable for an attack via excessive
number of events sent by the backend. Fix that by using lateeoi event
channels.

This is part of XSA-391

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/block/xen-blkfront.c |   15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

--- a/drivers/block/xen-blkfront.c
+++ b/drivers/block/xen-blkfront.c
@@ -1555,9 +1555,12 @@ static irqreturn_t blkif_interrupt(int i
 	struct blkfront_ring_info *rinfo = (struct blkfront_ring_info *)dev_id;
 	struct blkfront_info *info = rinfo->dev_info;
 	int error;
+	unsigned int eoiflag = XEN_EOI_FLAG_SPURIOUS;
 
-	if (unlikely(info->connected != BLKIF_STATE_CONNECTED))
+	if (unlikely(info->connected != BLKIF_STATE_CONNECTED)) {
+		xen_irq_lateeoi(irq, XEN_EOI_FLAG_SPURIOUS);
 		return IRQ_HANDLED;
+	}
 
 	spin_lock_irqsave(&rinfo->ring_lock, flags);
  again:
@@ -1573,6 +1576,8 @@ static irqreturn_t blkif_interrupt(int i
 		unsigned long id;
 		unsigned int op;
 
+		eoiflag = 0;
+
 		RING_COPY_RESPONSE(&rinfo->ring, i, &bret);
 		id = bret.id;
 
@@ -1684,6 +1689,8 @@ static irqreturn_t blkif_interrupt(int i
 
 	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
 
+	xen_irq_lateeoi(irq, eoiflag);
+
 	return IRQ_HANDLED;
 
  err:
@@ -1691,6 +1698,8 @@ static irqreturn_t blkif_interrupt(int i
 
 	spin_unlock_irqrestore(&rinfo->ring_lock, flags);
 
+	/* No EOI in order to avoid further interrupts. */
+
 	pr_alert("%s disabled for further use\n", info->gd->disk_name);
 	return IRQ_HANDLED;
 }
@@ -1730,8 +1739,8 @@ static int setup_blkring(struct xenbus_d
 	if (err)
 		goto fail;
 
-	err = bind_evtchn_to_irqhandler(rinfo->evtchn, blkif_interrupt, 0,
-					"blkif", rinfo);
+	err = bind_evtchn_to_irqhandler_lateeoi(rinfo->evtchn, blkif_interrupt,
+						0, "blkif", rinfo);
 	if (err <= 0) {
 		xenbus_dev_fatal(dev, err,
 				 "bind_evtchn_to_irqhandler failed");