xen/netback: Ensure protocol headers don't fall in the non-linear area

In some cases, the frontend may send a packet where the protocol headers
are spread across multiple slots. This would result in netback creating
an skb where the protocol headers spill over into the non-linear area.
Some drivers and NICs don't handle this properly resulting in an
interface reset or worse.

This issue was introduced by the removal of an unconditional skb pull in
the tx path to improve performance.  Fix this without reintroducing the
pull by setting up grant copy ops for as many slots as needed to reach
the XEN_NETBACK_TX_COPY_LEN size. Adjust the rest of the code to handle
multiple copy operations per skb.

This is XSA-423 / CVE-2022-3643.

Fixes: 7e5d7753 ("xen-netback: remove unconditional __pskb_pull_tail() in guest Tx path")
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/net/xen-netback/netback.c

@@ -332,10 +332,13 @@ static int xenvif_count_requests(struct xenvif_queue *queue,
 
 
 struct xenvif_tx_cb {
-	u16 pending_idx;
+	u16 copy_pending_idx[XEN_NETBK_LEGACY_SLOTS_MAX + 1];
+	u8 copy_count;
 };
 
 #define XENVIF_TX_CB(skb) ((struct xenvif_tx_cb *)(skb)->cb)
+#define copy_pending_idx(skb, i) (XENVIF_TX_CB(skb)->copy_pending_idx[i])
+#define copy_count(skb) (XENVIF_TX_CB(skb)->copy_count)
 
 static inline void xenvif_tx_create_map_op(struct xenvif_queue *queue,
 					   u16 pending_idx,
@@ -370,31 +373,93 @@ static inline struct sk_buff *xenvif_alloc_skb(unsigned int size)
 	return skb;
 }
 
-static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif_queue *queue,
+static void xenvif_get_requests(struct xenvif_queue *queue,
 				struct sk_buff *skb,
-							struct xen_netif_tx_request *txp,
-							struct gnttab_map_grant_ref *gop,
+				struct xen_netif_tx_request *first,
+				struct xen_netif_tx_request *txfrags,
+			        unsigned *copy_ops,
+			        unsigned *map_ops,
 				unsigned int frag_overflow,
-							struct sk_buff *nskb)
+				struct sk_buff *nskb,
+				unsigned int extra_count,
+				unsigned int data_len)
 {
 	struct skb_shared_info *shinfo = skb_shinfo(skb);
 	skb_frag_t *frags = shinfo->frags;
-	u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
-	int start;
+	u16 pending_idx;
 	pending_ring_idx_t index;
 	unsigned int nr_slots;
+	struct gnttab_copy *cop = queue->tx_copy_ops + *copy_ops;
+	struct gnttab_map_grant_ref *gop = queue->tx_map_ops + *map_ops;
+	struct xen_netif_tx_request *txp = first;
 
-	nr_slots = shinfo->nr_frags;
+	nr_slots = shinfo->nr_frags + 1;
 
-	/* Skip first skb fragment if it is on same page as header fragment. */
-	start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);
+	copy_count(skb) = 0;
 
-	for (shinfo->nr_frags = start; shinfo->nr_frags < nr_slots;
-	     shinfo->nr_frags++, txp++, gop++) {
+	/* Create copy ops for exactly data_len bytes into the skb head. */
+	__skb_put(skb, data_len);
+	while (data_len > 0) {
+		int amount = data_len > txp->size ? txp->size : data_len;
+
+		cop->source.u.ref = txp->gref;
+		cop->source.domid = queue->vif->domid;
+		cop->source.offset = txp->offset;
+
+		cop->dest.domid = DOMID_SELF;
+		cop->dest.offset = (offset_in_page(skb->data +
+						   skb_headlen(skb) -
+						   data_len)) & ~XEN_PAGE_MASK;
+		cop->dest.u.gmfn = virt_to_gfn(skb->data + skb_headlen(skb)
+				               - data_len);
+
+		cop->len = amount;
+		cop->flags = GNTCOPY_source_gref;
+
+		index = pending_index(queue->pending_cons);
+		pending_idx = queue->pending_ring[index];
+		callback_param(queue, pending_idx).ctx = NULL;
+		copy_pending_idx(skb, copy_count(skb)) = pending_idx;
+		copy_count(skb)++;
+
+		cop++;
+		data_len -= amount;
+
+		if (amount == txp->size) {
+			/* The copy op covered the full tx_request */
+
+			memcpy(&queue->pending_tx_info[pending_idx].req,
+			       txp, sizeof(*txp));
+			queue->pending_tx_info[pending_idx].extra_count =
+				(txp == first) ? extra_count : 0;
+
+			if (txp == first)
+				txp = txfrags;
+			else
+				txp++;
+			queue->pending_cons++;
+			nr_slots--;
+		} else {
+			/* The copy op partially covered the tx_request.
+			 * The remainder will be mapped.
+			 */
+			txp->offset += amount;
+			txp->size -= amount;
+		}
+	}
+
+	for (shinfo->nr_frags = 0; shinfo->nr_frags < nr_slots;
+	     shinfo->nr_frags++, gop++) {
 		index = pending_index(queue->pending_cons++);
 		pending_idx = queue->pending_ring[index];
-		xenvif_tx_create_map_op(queue, pending_idx, txp, 0, gop);
+		xenvif_tx_create_map_op(queue, pending_idx, txp,
+				        txp == first ? extra_count : 0, gop);
 		frag_set_pending_idx(&frags[shinfo->nr_frags], pending_idx);
+
+		if (txp == first)
+			txp = txfrags;
+		else
+			txp++;
 	}
 
 	if (frag_overflow) {
@@ -415,7 +480,8 @@ static struct gnttab_map_grant_ref *xenvif_get_requests(struct xenvif_queue *que
 		skb_shinfo(skb)->frag_list = nskb;
 	}
 
-	return gop;
+	(*copy_ops) = cop - queue->tx_copy_ops;
+	(*map_ops) = gop - queue->tx_map_ops;
 }
 
 static inline void xenvif_grant_handle_set(struct xenvif_queue *queue,
@@ -451,7 +517,7 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 			       struct gnttab_copy **gopp_copy)
 {
 	struct gnttab_map_grant_ref *gop_map = *gopp_map;
-	u16 pending_idx = XENVIF_TX_CB(skb)->pending_idx;
+	u16 pending_idx;
 	/* This always points to the shinfo of the skb being checked, which
 	 * could be either the first or the one on the frag_list
 	 */
@@ -462,12 +528,24 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 	struct skb_shared_info *first_shinfo = NULL;
 	int nr_frags = shinfo->nr_frags;
 	const bool sharedslot = nr_frags &&
-				frag_get_pending_idx(&shinfo->frags[0]) == pending_idx;
+				frag_get_pending_idx(&shinfo->frags[0]) ==
+				    copy_pending_idx(skb, copy_count(skb) - 1);
 	int i, err;
 
+	for (i = 0; i < copy_count(skb); i++) {
+		int newerr;
+
 		/* Check status of header. */
-	err = (*gopp_copy)->status;
-	if (unlikely(err)) {
+		pending_idx = copy_pending_idx(skb, i);
+
+		newerr = (*gopp_copy)->status;
+		if (likely(!newerr)) {
+			/* The first frag might still have this slot mapped */
+			if (i < copy_count(skb) - 1 || !sharedslot)
+				xenvif_idx_release(queue, pending_idx,
+						   XEN_NETIF_RSP_OKAY);
+		} else {
+			err = newerr;
 			if (net_ratelimit())
 				netdev_dbg(queue->vif->dev,
 					   "Grant copy of header failed! status: %d pending_idx: %u ref: %u\n",
@@ -475,11 +553,12 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 					   pending_idx,
 					   (*gopp_copy)->source.u.ref);
 			/* The first frag might still have this slot mapped */
-		if (!sharedslot)
+			if (i < copy_count(skb) - 1 || !sharedslot)
 				xenvif_idx_release(queue, pending_idx,
 						   XEN_NETIF_RSP_ERROR);
 		}
 		(*gopp_copy)++;
+	}
 
 check_frags:
 	for (i = 0; i < nr_frags; i++, gop_map++) {
@@ -526,14 +605,6 @@ static int xenvif_tx_check_gop(struct xenvif_queue *queue,
 		if (err)
 			continue;
 
-		/* First error: if the header haven't shared a slot with the
-		 * first frag, release it as well.
-		 */
-		if (!sharedslot)
-			xenvif_idx_release(queue,
-					   XENVIF_TX_CB(skb)->pending_idx,
-					   XEN_NETIF_RSP_OKAY);
-
 		/* Invalidate preceding fragments of this skb. */
 		for (j = 0; j < i; j++) {
 			pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
@@ -803,7 +874,6 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 				     unsigned *copy_ops,
 				     unsigned *map_ops)
 {
-	struct gnttab_map_grant_ref *gop = queue->tx_map_ops;
 	struct sk_buff *skb, *nskb;
 	int ret;
 	unsigned int frag_overflow;
@@ -885,8 +955,12 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 			continue;
 		}
 
+		data_len = (txreq.size > XEN_NETBACK_TX_COPY_LEN) ?
+			XEN_NETBACK_TX_COPY_LEN : txreq.size;
+
 		ret = xenvif_count_requests(queue, &txreq, extra_count,
 					    txfrags, work_to_do);
+
 		if (unlikely(ret < 0))
 			break;
 
@@ -912,9 +986,8 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 		index = pending_index(queue->pending_cons);
 		pending_idx = queue->pending_ring[index];
 
-		data_len = (txreq.size > XEN_NETBACK_TX_COPY_LEN &&
-			    ret < XEN_NETBK_LEGACY_SLOTS_MAX) ?
-			XEN_NETBACK_TX_COPY_LEN : txreq.size;
+		if (ret >= XEN_NETBK_LEGACY_SLOTS_MAX - 1 && data_len < txreq.size)
+			data_len = txreq.size;
 
 		skb = xenvif_alloc_skb(data_len);
 		if (unlikely(skb == NULL)) {
@@ -925,8 +998,6 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 		}
 
 		skb_shinfo(skb)->nr_frags = ret;
-		if (data_len < txreq.size)
-			skb_shinfo(skb)->nr_frags++;
 		/* At this point shinfo->nr_frags is in fact the number of
 		 * slots, which can be as large as XEN_NETBK_LEGACY_SLOTS_MAX.
 		 */
@@ -988,54 +1059,19 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
 					     type);
 		}
 
-		XENVIF_TX_CB(skb)->pending_idx = pending_idx;
-
-		__skb_put(skb, data_len);
-		queue->tx_copy_ops[*copy_ops].source.u.ref = txreq.gref;
-		queue->tx_copy_ops[*copy_ops].source.domid = queue->vif->domid;
-		queue->tx_copy_ops[*copy_ops].source.offset = txreq.offset;
-
-		queue->tx_copy_ops[*copy_ops].dest.u.gmfn =
-			virt_to_gfn(skb->data);
-		queue->tx_copy_ops[*copy_ops].dest.domid = DOMID_SELF;
-		queue->tx_copy_ops[*copy_ops].dest.offset =
-			offset_in_page(skb->data) & ~XEN_PAGE_MASK;
-
-		queue->tx_copy_ops[*copy_ops].len = data_len;
-		queue->tx_copy_ops[*copy_ops].flags = GNTCOPY_source_gref;
-
-		(*copy_ops)++;
-
-		if (data_len < txreq.size) {
-			frag_set_pending_idx(&skb_shinfo(skb)->frags[0],
-					     pending_idx);
-			xenvif_tx_create_map_op(queue, pending_idx, &txreq,
-						extra_count, gop);
-			gop++;
-		} else {
-			frag_set_pending_idx(&skb_shinfo(skb)->frags[0],
-					     INVALID_PENDING_IDX);
-			memcpy(&queue->pending_tx_info[pending_idx].req,
-			       &txreq, sizeof(txreq));
-			queue->pending_tx_info[pending_idx].extra_count =
-				extra_count;
-		}
-
-		queue->pending_cons++;
-
-		gop = xenvif_get_requests(queue, skb, txfrags, gop,
-				          frag_overflow, nskb);
+		xenvif_get_requests(queue, skb, &txreq, txfrags, copy_ops,
+				    map_ops, frag_overflow, nskb, extra_count,
+				    data_len);
 
 		__skb_queue_tail(&queue->tx_queue, skb);
 
 		queue->tx.req_cons = idx;
 
-		if (((gop-queue->tx_map_ops) >= ARRAY_SIZE(queue->tx_map_ops)) ||
+		if ((*map_ops >= ARRAY_SIZE(queue->tx_map_ops)) ||
 		    (*copy_ops >= ARRAY_SIZE(queue->tx_copy_ops)))
 			break;
 	}
 
-	(*map_ops) = gop - queue->tx_map_ops;
 	return;
 }
 
@@ -1114,9 +1150,8 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
 	while ((skb = __skb_dequeue(&queue->tx_queue)) != NULL) {
 		struct xen_netif_tx_request *txp;
 		u16 pending_idx;
-		unsigned data_len;
 
-		pending_idx = XENVIF_TX_CB(skb)->pending_idx;
+		pending_idx = copy_pending_idx(skb, 0);
 		txp = &queue->pending_tx_info[pending_idx].req;
 
 		/* Check the remap error code. */
@@ -1135,18 +1170,6 @@ static int xenvif_tx_submit(struct xenvif_queue *queue)
 			continue;
 		}
 
-		data_len = skb->len;
-		callback_param(queue, pending_idx).ctx = NULL;
-		if (data_len < txp->size) {
-			/* Append the packet payload as a fragment. */
-			txp->offset += data_len;
-			txp->size -= data_len;
-		} else {
-			/* Schedule a response immediately. */
-			xenvif_idx_release(queue, pending_idx,
-					   XEN_NETIF_RSP_OKAY);
-		}
-
 		if (txp->flags & XEN_NETTXF_csum_blank)
 			skb->ip_summed = CHECKSUM_PARTIAL;
 		else if (txp->flags & XEN_NETTXF_data_validated)
@@ -1332,7 +1355,7 @@ static inline void xenvif_tx_dealloc_action(struct xenvif_queue *queue)
 /* Called after netfront has transmitted */
 int xenvif_tx_action(struct xenvif_queue *queue, int budget)
 {
-	unsigned nr_mops, nr_cops = 0;
+	unsigned nr_mops = 0, nr_cops = 0;
 	int work_done, ret;
 
 	if (unlikely(!tx_work_todo(queue)))