[PATCH untested] vhost: order avail ring reads after index updates

[Michael S. Tsirkin]

vhost_get_vq_desc (correctly) uses smp_rmb to order
avail ring reads after index reads.
However, over time we added two more places that read the
index and do not bother with barriers.
Since vhost_get_vq_desc when it was written assumed it is the
only reader when it sees a new index value is cached
it does not bother with a barrier either, as a result,
on the nvidia-gracehopper platform (arm64) available ring
entry reads have been observed bypassing ring reads, causing
a ring corruption.

To fix, factor out the correct index access code from vhost_get_vq_desc.
As a side benefit, we also validate the index on all paths now, which
will hopefully help catch future errors earlier.

Note: current code is inconsistent in how it handles errors:
some places treat it as an empty ring, others - non empty.
This patch does not attempt to change the existing behaviour.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Gavin Shan <gshan@xxxxxxxxxx>
Reported-by: Will Deacon <will@xxxxxxxxxx>
Suggested-by: Will Deacon <will@xxxxxxxxxx>
Fixes: 275bf960ac69 ("vhost: better detection of available buffers")
Cc: "Jason Wang" <jasowang@xxxxxxxxxx>
Fixes: d3bb267bbdcb ("vhost: cache avail index in vhost_enable_notify()")
Cc: "Stefano Garzarella" <sgarzare@xxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---

I think it's better to bite the bullet and clean up the code.
Note: this is still only built, not tested.
Gavin could you help test please?
Especially on the arm platform you have?

Will thanks so much for finding this race!


 drivers/vhost/vhost.c | 80 +++++++++++++++++++++++--------------------
 1 file changed, 42 insertions(+), 38 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 045f666b4f12..26b70b1fd9ff 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1290,10 +1290,38 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
 		mutex_unlock(&d->vqs[i]->mutex);
 }
 
-static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
-				      __virtio16 *idx)
+static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
 {
-	return vhost_get_avail(vq, *idx, &vq->avail->idx);
+	__virtio16 idx;
+	u16 avail_idx;
+	int r = vhost_get_avail(vq, idx, &vq->avail->idx);
+
+	if (unlikely(r < 0)) {
+		vq_err(vq, "Failed to access avail idx at %p: %d\n",
+		       &vq->avail->idx, r);
+		return -EFAULT;
+	}
+
+	avail_idx = vhost16_to_cpu(vq, idx);
+
+	/* Check it isn't doing very strange things with descriptor numbers. */
+	if (unlikely((u16)(avail_idx - vq->last_avail_idx) > vq->num)) {
+		vq_err(vq, "Guest moved used index from %u to %u",
+		       vq->last_avail_idx, vq->avail_idx);
+		return -EFAULT;
+	}
+
+	/* Nothing new? We are done. */
+	if (avail_idx == vq->avail_idx)
+		return 0;
+
+	vq->avail_idx = avail_idx;
+
+	/* We updated vq->avail_idx so we need a memory barrier between
+	 * the index read above and the caller reading avail ring entries.
+	 */
+	smp_rmb();
+	return 1;
 }
 
 static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
@@ -2498,38 +2526,21 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 {
 	struct vring_desc desc;
 	unsigned int i, head, found = 0;
-	u16 last_avail_idx;
-	__virtio16 avail_idx;
+	u16 last_avail_idx = vq->last_avail_idx;
 	__virtio16 ring_head;
 	int ret, access;
 
-	/* Check it isn't doing very strange things with descriptor numbers. */
-	last_avail_idx = vq->last_avail_idx;
 
 	if (vq->avail_idx == vq->last_avail_idx) {
-		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
-			vq_err(vq, "Failed to access avail idx at %p\n",
-				&vq->avail->idx);
-			return -EFAULT;
-		}
-		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
-
-		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
-			vq_err(vq, "Guest moved used index from %u to %u",
-				last_avail_idx, vq->avail_idx);
-			return -EFAULT;
-		}
+		ret = vhost_get_avail_idx(vq);
+		if (unlikely(ret < 0))
+			return ret;
 
 		/* If there's nothing new since last we looked, return
 		 * invalid.
 		 */
-		if (vq->avail_idx == last_avail_idx)
+		if (!ret)
 			return vq->num;
-
-		/* Only get avail ring entries after they have been
-		 * exposed by guest.
-		 */
-		smp_rmb();
 	}
 
 	/* Grab the next descriptor number they're advertising, and increment
@@ -2790,25 +2801,21 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
 /* return true if we're sure that avaiable ring is empty */
 bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
-	__virtio16 avail_idx;
 	int r;
 
 	if (vq->avail_idx != vq->last_avail_idx)
 		return false;
 
-	r = vhost_get_avail_idx(vq, &avail_idx);
-	if (unlikely(r))
-		return false;
-	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
+	r = vhost_get_avail_idx(vq);
 
-	return vq->avail_idx == vq->last_avail_idx;
+	/* Note: we treat error as non-empty here */
+	return r == 0;
 }
 EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
 
 /* OK, now we need to know about added descriptors. */
 bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 {
-	__virtio16 avail_idx;
 	int r;
 
 	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
@@ -2832,13 +2839,10 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
 	/* They could have slipped one in as we were doing that: make
 	 * sure it's written, then check again. */
 	smp_mb();
-	r = vhost_get_avail_idx(vq, &avail_idx);
-	if (r) {
-		vq_err(vq, "Failed to check avail idx at %p: %d\n",
-		       &vq->avail->idx, r);
+	r = vhost_get_avail_idx(vq);
+	/* Note: we treat error as empty here */
+	if (r < 0)
 		return false;
-	}
-	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
 
 	return vq->avail_idx != vq->last_avail_idx;
 }
-- 
MST