virtio_ring : keep used_wrap_counter in vq->last_used_idx

the used_wrap_counter and the vq->last_used_idx may get
out of sync if they are separate assignment，and interrupt
might use an incorrect value to check for the used index.

for example:OOB access
ksoftirqd may consume the packet and it will call:
virtnet_poll
	-->virtnet_receive
		-->virtqueue_get_buf_ctx
			-->virtqueue_get_buf_ctx_packed
and in virtqueue_get_buf_ctx_packed:

vq->last_used_idx += vq->packed.desc_state[id].num;
if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
         vq->last_used_idx -= vq->packed.vring.num;
         vq->packed.used_wrap_counter ^= 1;
}

if at the same time, there comes a vring interrupt，in vring_interrupt:
we will call:
vring_interrupt
	-->more_used
		-->more_used_packed
			-->is_used_desc_packed
in is_used_desc_packed, the last_used_idx maybe >= vq->packed.vring.num.
so this could case a memory out of bounds bug.

this patch is to keep the used_wrap_counter in vq->last_used_idx
so we can get the correct value to check for used index in interrupt.

v3->v4:
- use READ_ONCE/WRITE_ONCE to get/set vq->last_used_idx

v2->v3:
- add inline function to get used_wrap_counter and last_used
- when use vq->last_used_idx, only read once
  if vq->last_used_idx is read twice, the values can be inconsistent.
- use last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR))
  to get the all bits below VRING_PACKED_EVENT_F_WRAP_CTR

v1->v2:
- reuse the VRING_PACKED_EVENT_F_WRAP_CTR
- Remove parameter judgment in is_used_desc_packed,
because it can't be illegal
Signed-off-by: huangjie.albert <huangjie.albert@bytedance.com>
Message-Id: <20220617020411.80367-1-huangjie.albert@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -111,7 +111,12 @@ struct vring_virtqueue {
 	/* Number we've added since last sync. */
 	unsigned int num_added;
 
-	/* Last used index we've seen. */
+	/* Last used index  we've seen.
+	 * for split ring, it just contains last used index
+	 * for packed ring:
+	 * bits up to VRING_PACKED_EVENT_F_WRAP_CTR include the last used index.
+	 * bits from VRING_PACKED_EVENT_F_WRAP_CTR include the used wrap counter.
+	 */
 	u16 last_used_idx;
 
 	/* Hint for event idx: already triggered no need to disable. */
@@ -154,9 +159,6 @@ struct vring_virtqueue {
 			/* Driver ring wrap counter. */
 			bool avail_wrap_counter;
 
-			/* Device ring wrap counter. */
-			bool used_wrap_counter;
-
 			/* Avail used flags. */
 			u16 avail_used_flags;
 
@@ -973,6 +975,15 @@ static struct virtqueue *vring_create_virtqueue_split(
 /*
  * Packed ring specific functions - *_packed().
  */
+static inline bool packed_used_wrap_counter(u16 last_used_idx)
+{
+	return !!(last_used_idx & (1 << VRING_PACKED_EVENT_F_WRAP_CTR));
+}
+
+static inline u16 packed_last_used(u16 last_used_idx)
+{
+	return last_used_idx & ~(-(1 << VRING_PACKED_EVENT_F_WRAP_CTR));
+}
 
 static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
 				     struct vring_desc_extra *extra)
@@ -1406,8 +1417,14 @@ static inline bool is_used_desc_packed(const struct vring_virtqueue *vq,
 
 static inline bool more_used_packed(const struct vring_virtqueue *vq)
 {
-	return is_used_desc_packed(vq, vq->last_used_idx,
-			vq->packed.used_wrap_counter);
+	u16 last_used;
+	u16 last_used_idx;
+	bool used_wrap_counter;
+
+	last_used_idx = READ_ONCE(vq->last_used_idx);
+	last_used = packed_last_used(last_used_idx);
+	used_wrap_counter = packed_used_wrap_counter(last_used_idx);
+	return is_used_desc_packed(vq, last_used, used_wrap_counter);
 }
 
 static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
@@ -1415,7 +1432,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 					  void **ctx)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 last_used, id;
+	u16 last_used, id, last_used_idx;
+	bool used_wrap_counter;
 	void *ret;
 
 	START_USE(vq);
@@ -1434,7 +1452,9 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 	/* Only get used elements after they have been exposed by host. */
 	virtio_rmb(vq->weak_barriers);
 
-	last_used = vq->last_used_idx;
+	last_used_idx = READ_ONCE(vq->last_used_idx);
+	used_wrap_counter = packed_used_wrap_counter(last_used_idx);
+	last_used = packed_last_used(last_used_idx);
 	id = le16_to_cpu(vq->packed.vring.desc[last_used].id);
 	*len = le32_to_cpu(vq->packed.vring.desc[last_used].len);
 
@@ -1451,12 +1471,15 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 	ret = vq->packed.desc_state[id].data;
 	detach_buf_packed(vq, id, ctx);
 
-	vq->last_used_idx += vq->packed.desc_state[id].num;
-	if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
-		vq->last_used_idx -= vq->packed.vring.num;
-		vq->packed.used_wrap_counter ^= 1;
+	last_used += vq->packed.desc_state[id].num;
+	if (unlikely(last_used >= vq->packed.vring.num)) {
+		last_used -= vq->packed.vring.num;
+		used_wrap_counter ^= 1;
 	}
 
+	last_used = (last_used | (used_wrap_counter << VRING_PACKED_EVENT_F_WRAP_CTR));
+	WRITE_ONCE(vq->last_used_idx, last_used);
+
 	/*
 	 * If we expect an interrupt for the next entry, tell host
 	 * by writing event index and flush out the write before
@@ -1465,9 +1488,7 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
 	if (vq->packed.event_flags_shadow == VRING_PACKED_EVENT_FLAG_DESC)
 		virtio_store_mb(vq->weak_barriers,
 				&vq->packed.vring.driver->off_wrap,
-				cpu_to_le16(vq->last_used_idx |
-					(vq->packed.used_wrap_counter <<
-					 VRING_PACKED_EVENT_F_WRAP_CTR)));
+				cpu_to_le16(vq->last_used_idx));
 
 	LAST_ADD_TIME_INVALID(vq);
 
@@ -1499,9 +1520,7 @@ static unsigned int virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
 
 	if (vq->event) {
 		vq->packed.vring.driver->off_wrap =
-			cpu_to_le16(vq->last_used_idx |
-				(vq->packed.used_wrap_counter <<
-				 VRING_PACKED_EVENT_F_WRAP_CTR));
+			cpu_to_le16(vq->last_used_idx);
 		/*
 		 * We need to update event offset and event wrap
 		 * counter first before updating event flags.
@@ -1518,8 +1537,7 @@ static unsigned int virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
 	}
 
 	END_USE(vq);
-	return vq->last_used_idx | ((u16)vq->packed.used_wrap_counter <<
-			VRING_PACKED_EVENT_F_WRAP_CTR);
+	return vq->last_used_idx;
 }
 
 static bool virtqueue_poll_packed(struct virtqueue *_vq, u16 off_wrap)
@@ -1537,7 +1555,7 @@ static bool virtqueue_poll_packed(struct virtqueue *_vq, u16 off_wrap)
 static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
-	u16 used_idx, wrap_counter;
+	u16 used_idx, wrap_counter, last_used_idx;
 	u16 bufs;
 
 	START_USE(vq);
@@ -1550,9 +1568,10 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
 	if (vq->event) {
 		/* TODO: tune this threshold */
 		bufs = (vq->packed.vring.num - vq->vq.num_free) * 3 / 4;
-		wrap_counter = vq->packed.used_wrap_counter;
+		last_used_idx = READ_ONCE(vq->last_used_idx);
+		wrap_counter = packed_used_wrap_counter(last_used_idx);
 
-		used_idx = vq->last_used_idx + bufs;
+		used_idx = packed_last_used(last_used_idx) + bufs;
 		if (used_idx >= vq->packed.vring.num) {
 			used_idx -= vq->packed.vring.num;
 			wrap_counter ^= 1;
@@ -1582,9 +1601,10 @@ static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
 	 */
 	virtio_mb(vq->weak_barriers);
 
-	if (is_used_desc_packed(vq,
-				vq->last_used_idx,
-				vq->packed.used_wrap_counter)) {
+	last_used_idx = READ_ONCE(vq->last_used_idx);
+	wrap_counter = packed_used_wrap_counter(last_used_idx);
+	used_idx = packed_last_used(last_used_idx);
+	if (is_used_desc_packed(vq, used_idx, wrap_counter)) {
 		END_USE(vq);
 		return false;
 	}
@@ -1689,7 +1709,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
 	vq->notify = notify;
 	vq->weak_barriers = weak_barriers;
 	vq->broken = true;
-	vq->last_used_idx = 0;
+	vq->last_used_idx = 0 | (1 << VRING_PACKED_EVENT_F_WRAP_CTR);
 	vq->event_triggered = false;
 	vq->num_added = 0;
 	vq->packed_ring = true;
@@ -1720,7 +1740,6 @@ static struct virtqueue *vring_create_virtqueue_packed(
 
 	vq->packed.next_avail_idx = 0;
 	vq->packed.avail_wrap_counter = 1;
-	vq->packed.used_wrap_counter = 1;
 	vq->packed.event_flags_shadow = 0;
 	vq->packed.avail_used_flags = 1 << VRING_PACKED_DESC_F_AVAIL;