vhost: move smp_rmb() into vhost_get_avail_idx()

All callers of vhost_get_avail_idx() use smp_rmb() to
order the available ring entry read and avail_idx read.

Make vhost_get_avail_idx() call smp_rmb() itself whenever the avail_idx
is accessed. This way, the callers don't need to worry about the memory
barrier. As a side benefit, we also validate the index on all paths now,
which will hopefully help prevent/catch earlier future bugs.

Note that current code is inconsistent in how the errors are handled.
They are treated as an empty ring in some places, but as non-empty
ring in other places. This patch doesn't attempt to change the existing
behaviour.

No functional change intended.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Message-Id: <20240429232748.642356-1-gshan@redhat.com>

drivers/vhost/vhost.c

@@ -1346,10 +1346,36 @@ 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;
+	int r;
+
+	r = vhost_get_avail(vq, idx, &vq->avail->idx);
+	if (unlikely(r < 0)) {
+		vq_err(vq, "Failed to access available index at %p (%d)\n",
+		       &vq->avail->idx, r);
+		return r;
+	}
+
+	/* Check it isn't doing very strange thing with available indexes */
+	vq->avail_idx = vhost16_to_cpu(vq, idx);
+	if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
+		vq_err(vq, "Invalid available index change from %u to %u",
+		       vq->last_avail_idx, vq->avail_idx);
+		return -EINVAL;
+	}
+
+	/* We're done if there is nothing new */
+	if (vq->avail_idx == vq->last_avail_idx)
+		return 0;
+
+	/*
+	 * 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,
@@ -2554,38 +2580,17 @@ 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 avail 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
@@ -2846,35 +2851,21 @@ 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);
-	if (vq->avail_idx != vq->last_avail_idx) {
-		/* Since we have updated avail_idx, the following
-		 * call to vhost_get_vq_desc() will read available
-		 * ring entries. Make sure that read happens after
-		 * the avail_idx read.
-		 */
-		smp_rmb();
-		return false;
-	}
+	r = vhost_get_avail_idx(vq);
 
-	return true;
+	/* 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))
@@ -2898,25 +2889,13 @@ 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);
-		return false;
-	}
 
-	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
-	if (vq->avail_idx != vq->last_avail_idx) {
-		/* Since we have updated avail_idx, the following
-		 * call to vhost_get_vq_desc() will read available
-		 * ring entries. Make sure that read happens after
-		 * the avail_idx read.
-		 */
-		smp_rmb();
-		return true;
-	}
+	r = vhost_get_avail_idx(vq);
+	/* Note: we treat error as empty here */
+	if (unlikely(r < 0))
+		return false;
 
-	return false;
+	return r;
 }
 EXPORT_SYMBOL_GPL(vhost_enable_notify);