[PATCH V5 0/7] Bundle fixes for Xen netfront /netback

[PATCH V5 0/7] Bundle fixes for Xen netfront /netback

[Wei Liu]

V5 of this series.

1-3 have been applied to DaveM's tree. Repost them for the people who don't
want to cherry-pick.

4 is incremental patch for 2.

5-7 are the real meat with previous comments addressed.


Wei.

[PATCH V5 1/7] xen-netfront: remove unused variable `extra'

[Wei Liu]

This variable is supposed to hold reference to the last extra_info in the
loop. However there is only type of extra info here and the loop to process
extra info is missing, so this variable is never used and causes confusion.

Remove it at the moment. We can add it back when necessary.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
---
 drivers/net/xen-netfront.c |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 7ffa43b..5527663 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -537,7 +537,6 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct netfront_info *np = netdev_priv(dev);
 	struct netfront_stats *stats = this_cpu_ptr(np->stats);
 	struct xen_netif_tx_request *tx;
-	struct xen_netif_extra_info *extra;
 	char *data = skb->data;
 	RING_IDX i;
 	grant_ref_t ref;
@@ -581,7 +580,6 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	tx->gref = np->grant_tx_ref[id] = ref;
 	tx->offset = offset;
 	tx->size = len;
-	extra = NULL;
 
 	tx->flags = 0;
 	if (skb->ip_summed == CHECKSUM_PARTIAL)
@@ -597,10 +595,7 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 		gso = (struct xen_netif_extra_info *)
 			RING_GET_REQUEST(&np->tx, ++i);
 
-		if (extra)
-			extra->flags |= XEN_NETIF_EXTRA_FLAG_MORE;
-		else
-			tx->flags |= XEN_NETTXF_extra_info;
+		tx->flags |= XEN_NETTXF_extra_info;
 
 		gso->u.gso.size = skb_shinfo(skb)->gso_size;
 		gso->u.gso.type = XEN_NETIF_GSO_TYPE_TCPV4;
@@ -609,7 +604,6 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 		gso->type = XEN_NETIF_EXTRA_TYPE_GSO;
 		gso->flags = 0;
-		extra = gso;
 	}
 
 	np->tx.req_prod_pvt = i + 1;
-- 
1.7.10.4

[PATCH V5 2/7] xen-netfront: frags -> slots in xennet_get_responses

[Wei Liu]

This function is in fact counting the ring slots required for responses.
Separate the concepts of ring slots and skb frags make the code clearer, as
now netfront and netback can have different MAX_SKB_FRAGS, slot and frag are
not mapped 1:1 any more.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
---
 drivers/net/xen-netfront.c |   18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 5527663..d9097a7 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -712,7 +712,7 @@ static int xennet_get_responses(struct netfront_info *np,
 	struct sk_buff *skb = xennet_get_rx_skb(np, cons);
 	grant_ref_t ref = xennet_get_rx_ref(np, cons);
 	int max = MAX_SKB_FRAGS + (rx->status <= RX_COPY_THRESHOLD);
-	int frags = 1;
+	int slots = 1;
 	int err = 0;
 	unsigned long ret;
 
@@ -756,27 +756,27 @@ next:
 		if (!(rx->flags & XEN_NETRXF_more_data))
 			break;
 
-		if (cons + frags == rp) {
+		if (cons + slots == rp) {
 			if (net_ratelimit())
-				dev_warn(dev, "Need more frags\n");
+				dev_warn(dev, "Need more slots\n");
 			err = -ENOENT;
 			break;
 		}
 
-		rx = RING_GET_RESPONSE(&np->rx, cons + frags);
-		skb = xennet_get_rx_skb(np, cons + frags);
-		ref = xennet_get_rx_ref(np, cons + frags);
-		frags++;
+		rx = RING_GET_RESPONSE(&np->rx, cons + slots);
+		skb = xennet_get_rx_skb(np, cons + slots);
+		ref = xennet_get_rx_ref(np, cons + slots);
+		slots++;
 	}
 
-	if (unlikely(frags > max)) {
+	if (unlikely(slots > max)) {
 		if (net_ratelimit())
 			dev_warn(dev, "Too many frags\n");
 		err = -E2BIG;
 	}
 
 	if (unlikely(err))
-		np->rx.rsp_cons = cons + frags;
+		np->rx.rsp_cons = cons + slots;
 
 	return err;
 }
-- 
1.7.10.4

[PATCH V5 3/7] xen-netback: remove skb in xen_netbk_alloc_page

[Wei Liu]

This variable is never used.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
 drivers/net/xen-netback/netback.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index da726a3..6e8e51a 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -948,7 +948,6 @@ static int netbk_count_requests(struct xenvif *vif,
 }
 
 static struct page *xen_netbk_alloc_page(struct xen_netbk *netbk,
-					 struct sk_buff *skb,
 					 u16 pending_idx)
 {
 	struct page *page;
@@ -982,7 +981,7 @@ static struct gnttab_copy *xen_netbk_get_requests(struct xen_netbk *netbk,
 
 		index = pending_index(netbk->pending_cons++);
 		pending_idx = netbk->pending_ring[index];
-		page = xen_netbk_alloc_page(netbk, skb, pending_idx);
+		page = xen_netbk_alloc_page(netbk, pending_idx);
 		if (!page)
 			goto err;
 
@@ -1387,7 +1386,7 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 		}
 
 		/* XXX could copy straight to head */
-		page = xen_netbk_alloc_page(netbk, skb, pending_idx);
+		page = xen_netbk_alloc_page(netbk, pending_idx);
 		if (!page) {
 			kfree_skb(skb);
 			netbk_tx_err(vif, &txreq, idx);
-- 
1.7.10.4

[PATCH V5 4/7] xen-netfront: frags -> slots in log message

[Wei Liu]

Also fix a typo in comment.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 drivers/net/xen-netfront.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index d9097a7..1bb2e20 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -735,7 +735,7 @@ static int xennet_get_responses(struct netfront_info *np,
 		/*
 		 * This definitely indicates a bug, either in this driver or in
 		 * the backend driver. In future this should flag the bad
-		 * situation to the system controller to reboot the backed.
+		 * situation to the system controller to reboot the backend.
 		 */
 		if (ref == GRANT_INVALID_REF) {
 			if (net_ratelimit())
@@ -771,7 +771,7 @@ next:
 
 	if (unlikely(slots > max)) {
 		if (net_ratelimit())
-			dev_warn(dev, "Too many frags\n");
+			dev_warn(dev, "Too many slots\n");
 		err = -E2BIG;
 	}
 
-- 
1.7.10.4

[PATCH V5 5/7] xen-netfront: reduce gso_max_size to account for max TCP header

[Wei Liu]

The maximum packet including header that can be handled by netfront / netback
wire format is 65535. Reduce gso_max_size accordingly.

Drop skb and print warning when skb->len > 65535. This can 1) save the effort
to send malformed packet to netback, 2) help spotting misconfiguration of
netfront in the future.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 drivers/net/xen-netfront.c       |   16 ++++++++++++++--
 include/xen/interface/io/netif.h |    1 +
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 1bb2e20..7c53d0c 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -36,7 +36,7 @@
 #include <linux/skbuff.h>
 #include <linux/ethtool.h>
 #include <linux/if_ether.h>
-#include <linux/tcp.h>
+#include <net/tcp.h>
 #include <linux/udp.h>
 #include <linux/moduleparam.h>
 #include <linux/mm.h>
@@ -547,6 +547,16 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned int len = skb_headlen(skb);
 	unsigned long flags;
 
+	/* If skb->len is too big for wire format, drop skb and alert
+	 * user about misconfiguration.
+	 */
+	if (unlikely(skb->len > XEN_NETIF_MAX_TX_SIZE)) {
+		net_alert_ratelimited(
+			"xennet: skb->len = %u, too big for wire format\n",
+			skb->len);
+		goto drop;
+	}
+
 	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
 		xennet_count_skb_frag_slots(skb);
 	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
@@ -1058,7 +1068,7 @@ err:
 
 static int xennet_change_mtu(struct net_device *dev, int mtu)
 {
-	int max = xennet_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
+	int max = xennet_can_sg(dev) ? 65535 - MAX_TCP_HEADER : ETH_DATA_LEN;
 
 	if (mtu > max)
 		return -EINVAL;
@@ -1362,6 +1372,8 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev)
 	SET_ETHTOOL_OPS(netdev, &xennet_ethtool_ops);
 	SET_NETDEV_DEV(netdev, &dev->dev);
 
+	netif_set_gso_max_size(netdev, 65535 - MAX_TCP_HEADER);
+
 	np->netdev = netdev;
 
 	netif_carrier_off(netdev);
diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
index 9dfc120..58fadca 100644
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -47,6 +47,7 @@
 #define _XEN_NETTXF_extra_info		(3)
 #define  XEN_NETTXF_extra_info		(1U<<_XEN_NETTXF_extra_info)
 
+#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
 struct xen_netif_tx_request {
     grant_ref_t gref;      /* Reference to buffer page */
     uint16_t offset;       /* Offset within buffer page */
-- 
1.7.10.4

[PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Wei Liu]

This patch tries to coalesce tx requests when constructing grant copy
structures. It enables netback to deal with situation when frontend's
MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.

With the help of coalescing, this patch tries to address two regressions and
avoid reopening the security hole in XSA-39.

Regression 1. The reduction of the number of supported ring entries (slots)
per packet (from 18 to 17). This regression has been around for some time but
remains unnoticed until XSA-39 security fix. This is fixed by coalescing
slots.

Regression 2. The XSA-39 security fix turning "too many frags" errors from
just dropping the packet to a fatal error and disabling the VIF. This is fixed
by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17)
which rules out false positive (using 18 slots is legit) and dropping packets
using 19 to `max_skb_slots` slots.

To avoid reopening security hole in XSA-39, frontend sending packet using more
than max_skb_slots is considered malicious.

The behavior of netback for packet is thus:

    1-18            slots: valid
   19-max_skb_slots slots: drop and respond with an error
   max_skb_slots+   slots: fatal error

max_skb_slots is configurable by admin, default value is 20.

Also change variable name from "frags" to "slots" in netbk_count_requests.

Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be
fixed with separate patch.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
 drivers/net/xen-netback/netback.c |  270 ++++++++++++++++++++++++++++++-------
 include/xen/interface/io/netif.h  |   18 +++
 2 files changed, 239 insertions(+), 49 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 6e8e51a..86d533a 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -47,11 +47,25 @@
 #include <asm/xen/hypercall.h>
 #include <asm/xen/page.h>
 
+/*
+ * This is the maximum slots a skb can have. If a guest sends a skb
+ * which exceeds this limit it is considered malicious.
+ */
+#define MAX_SKB_SLOTS_DEFAULT 20
+static unsigned int max_skb_slots = MAX_SKB_SLOTS_DEFAULT;
+module_param(max_skb_slots, uint, 0444);
+
+typedef unsigned int pending_ring_idx_t;
+#define INVALID_PENDING_RING_IDX (~0U)
+
 struct pending_tx_info {
-	struct xen_netif_tx_request req;
+	struct xen_netif_tx_request req; /* coalesced tx request  */
 	struct xenvif *vif;
+	pending_ring_idx_t head; /* head != INVALID_PENDING_RING_IDX
+				  * if it is head of one or more tx
+				  * reqs
+				  */
 };
-typedef unsigned int pending_ring_idx_t;
 
 struct netbk_rx_meta {
 	int id;
@@ -102,7 +116,11 @@ struct xen_netbk {
 	atomic_t netfront_count;
 
 	struct pending_tx_info pending_tx_info[MAX_PENDING_REQS];
-	struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS];
+	/* Coalescing tx requests before copying makes number of grant
+	 * copy ops greater or equal to number of slots required. In
+	 * worst case a tx request consumes 2 gnttab_copy.
+	 */
+	struct gnttab_copy tx_copy_ops[2*MAX_PENDING_REQS];
 
 	u16 pending_ring[MAX_PENDING_REQS];
 
@@ -118,6 +136,16 @@ struct xen_netbk {
 static struct xen_netbk *xen_netbk;
 static int xen_netbk_group_nr;
 
+/*
+ * If head != INVALID_PENDING_IDX, it means this tx request is head of
+ * one or more merged tx requests, otherwise it is the continuation of
+ * previous tx request.
+ */
+static inline int pending_tx_is_head(struct xen_netbk *netbk, RING_IDX idx)
+{
+	return netbk->pending_tx_info[idx].head != INVALID_PENDING_RING_IDX;
+}
+
 void xen_netbk_add_xenvif(struct xenvif *vif)
 {
 	int i;
@@ -250,6 +278,7 @@ static int max_required_rx_slots(struct xenvif *vif)
 {
 	int max = DIV_ROUND_UP(vif->dev->mtu, PAGE_SIZE);
 
+	/* XXX FIXME: RX path dependent on MAX_SKB_FRAGS */
 	if (vif->can_sg || vif->gso || vif->gso_prefix)
 		max += MAX_SKB_FRAGS + 1; /* extra_info + frags */
 
@@ -657,6 +686,7 @@ static void xen_netbk_rx_action(struct xen_netbk *netbk)
 		__skb_queue_tail(&rxq, skb);
 
 		/* Filled the batch queue? */
+		/* XXX FIXME: RX path dependent on MAX_SKB_FRAGS */
 		if (count + MAX_SKB_FRAGS >= XEN_NETIF_RX_RING_SIZE)
 			break;
 	}
@@ -904,47 +934,78 @@ static void netbk_fatal_tx_err(struct xenvif *vif)
 
 static int netbk_count_requests(struct xenvif *vif,
 				struct xen_netif_tx_request *first,
+				RING_IDX first_idx,
 				struct xen_netif_tx_request *txp,
 				int work_to_do)
 {
 	RING_IDX cons = vif->tx.req_cons;
-	int frags = 0;
+	int slots = 0;
+	int drop_err = 0;
 
 	if (!(first->flags & XEN_NETTXF_more_data))
 		return 0;
 
 	do {
-		if (frags >= work_to_do) {
-			netdev_err(vif->dev, "Need more frags\n");
+		if (slots >= work_to_do) {
+			netdev_err(vif->dev,
+				   "Asked for %d slots but exceeds this limit\n",
+				   work_to_do);
 			netbk_fatal_tx_err(vif);
 			return -ENODATA;
 		}
 
-		if (unlikely(frags >= MAX_SKB_FRAGS)) {
-			netdev_err(vif->dev, "Too many frags\n");
+		/* This guest is really using too many slots and
+		 * considered malicious.
+		 */
+		if (unlikely(slots >= max_skb_slots)) {
+			netdev_err(vif->dev,
+				   "Malicious frontend using %d slots, threshold %u\n",
+				   slots, max_skb_slots);
 			netbk_fatal_tx_err(vif);
 			return -E2BIG;
 		}
 
-		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
+		/* Xen network protocol had implicit dependency on
+		 * MAX_SKB_FRAGS. XEN_NETIF_NR_SLOTS_MIN is set to the
+		 * historical MAX_SKB_FRAGS value 18 to honor the same
+		 * behavior as before. Any packet using more than 18
+		 * slots but less than max_skb_slots slots is dropped
+		 */
+		if (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN) {
+			if (net_ratelimit())
+				netdev_dbg(vif->dev,
+					   "Too many slots (%d) exceeding limit (%d), dropping packet\n",
+					   slots, XEN_NETIF_NR_SLOTS_MIN);
+			drop_err = -E2BIG;
+		}
+
+		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
 		       sizeof(*txp));
 		if (txp->size > first->size) {
-			netdev_err(vif->dev, "Frag is bigger than frame.\n");
+			netdev_err(vif->dev,
+				   "Invalid tx request, slot size %u > remaining size %u\n",
+				   txp->size, first->size);
 			netbk_fatal_tx_err(vif);
 			return -EIO;
 		}
 
 		first->size -= txp->size;
-		frags++;
+		slots++;
 
 		if (unlikely((txp->offset + txp->size) > PAGE_SIZE)) {
-			netdev_err(vif->dev, "txp->offset: %x, size: %u\n",
+			netdev_err(vif->dev, "Cross page boundary, txp->offset: %x, size: %u\n",
 				 txp->offset, txp->size);
 			netbk_fatal_tx_err(vif);
 			return -EINVAL;
 		}
 	} while ((txp++)->flags & XEN_NETTXF_more_data);
-	return frags;
+
+	if (drop_err) {
+		netbk_tx_err(vif, first, first_idx + slots);
+		return drop_err;
+	}
+
+	return slots;
 }
 
 static struct page *xen_netbk_alloc_page(struct xen_netbk *netbk,
@@ -968,48 +1029,114 @@ static struct gnttab_copy *xen_netbk_get_requests(struct xen_netbk *netbk,
 	struct skb_shared_info *shinfo = skb_shinfo(skb);
 	skb_frag_t *frags = shinfo->frags;
 	u16 pending_idx = *((u16 *)skb->data);
-	int i, start;
+	u16 head_idx = 0;
+	int slot, start;
+	struct page *page;
+	pending_ring_idx_t index, start_idx = 0;
+	uint16_t dst_offset;
+	unsigned int nr_slots;
+	struct pending_tx_info *first = NULL;
+
+	/* At this point shinfo->nr_frags is in fact the number of
+	 * slots, which can be as large as XEN_NETIF_NR_SLOTS_MIN.
+	 */
+	nr_slots = shinfo->nr_frags;
 
 	/* Skip first skb fragment if it is on same page as header fragment. */
 	start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);
 
-	for (i = start; i < shinfo->nr_frags; i++, txp++) {
-		struct page *page;
-		pending_ring_idx_t index;
+	/* Coalesce tx requests, at this point the packet passed in
+	 * should be <= 64K. Any packets larger than 64K have been
+	 * handled in netbk_count_requests().
+	 */
+	for (shinfo->nr_frags = slot = start; slot < nr_slots;
+	     shinfo->nr_frags++) {
 		struct pending_tx_info *pending_tx_info =
 			netbk->pending_tx_info;
 
-		index = pending_index(netbk->pending_cons++);
-		pending_idx = netbk->pending_ring[index];
-		page = xen_netbk_alloc_page(netbk, pending_idx);
+		page = alloc_page(GFP_KERNEL|__GFP_COLD);
 		if (!page)
 			goto err;
 
-		gop->source.u.ref = txp->gref;
-		gop->source.domid = vif->domid;
-		gop->source.offset = txp->offset;
-
-		gop->dest.u.gmfn = virt_to_mfn(page_address(page));
-		gop->dest.domid = DOMID_SELF;
-		gop->dest.offset = txp->offset;
-
-		gop->len = txp->size;
-		gop->flags = GNTCOPY_source_gref;
+		dst_offset = 0;
+		first = NULL;
+		while (dst_offset < PAGE_SIZE && slot < nr_slots) {
+			gop->flags = GNTCOPY_source_gref;
+
+			gop->source.u.ref = txp->gref;
+			gop->source.domid = vif->domid;
+			gop->source.offset = txp->offset;
+
+			gop->dest.domid = DOMID_SELF;
+
+			gop->dest.offset = dst_offset;
+			gop->dest.u.gmfn = virt_to_mfn(page_address(page));
+
+			if (dst_offset + txp->size > PAGE_SIZE) {
+				/* This page can only merge a portion
+				 * of tx request. Do not increment any
+				 * pointer / counter here. The txp
+				 * will be dealt with in future
+				 * rounds, eventually hitting the
+				 * `else` branch.
+				 */
+				gop->len = PAGE_SIZE - dst_offset;
+				txp->offset += gop->len;
+				txp->size -= gop->len;
+				dst_offset += gop->len; /* quit loop */
+			} else {
+				/* This tx request can be merged in the page */
+				gop->len = txp->size;
+				dst_offset += gop->len;
+
+				index = pending_index(netbk->pending_cons++);
+
+				pending_idx = netbk->pending_ring[index];
+
+				memcpy(&pending_tx_info[pending_idx].req, txp,
+				       sizeof(*txp));
+				xenvif_get(vif);
+
+				pending_tx_info[pending_idx].vif = vif;
+
+				/* Poison these fields, corresponding
+				 * fields for head tx req will be set
+				 * to correct values after the loop.
+				 */
+				netbk->mmap_pages[pending_idx] = (void *)(~0UL);
+				pending_tx_info[pending_idx].head =
+					INVALID_PENDING_RING_IDX;
+
+				if (!first) {
+					first = &pending_tx_info[pending_idx];
+					start_idx = index;
+					head_idx = pending_idx;
+				}
+
+				txp++;
+				slot++;
+			}
 
-		gop++;
+			gop++;
+		}
 
-		memcpy(&pending_tx_info[pending_idx].req, txp, sizeof(*txp));
-		xenvif_get(vif);
-		pending_tx_info[pending_idx].vif = vif;
-		frag_set_pending_idx(&frags[i], pending_idx);
+		first->req.offset = 0;
+		first->req.size = dst_offset;
+		first->head = start_idx;
+		set_page_ext(page, netbk, head_idx);
+		netbk->mmap_pages[head_idx] = page;
+		frag_set_pending_idx(&frags[shinfo->nr_frags], head_idx);
 	}
 
+	BUG_ON(shinfo->nr_frags > MAX_SKB_FRAGS);
+
 	return gop;
 err:
 	/* Unwind, freeing all pages and sending error responses. */
-	while (i-- > start) {
-		xen_netbk_idx_release(netbk, frag_get_pending_idx(&frags[i]),
-				      XEN_NETIF_RSP_ERROR);
+	while (shinfo->nr_frags-- > start) {
+		xen_netbk_idx_release(netbk,
+				frag_get_pending_idx(&frags[shinfo->nr_frags]),
+				XEN_NETIF_RSP_ERROR);
 	}
 	/* The head too, if necessary. */
 	if (start)
@@ -1025,8 +1152,10 @@ static int xen_netbk_tx_check_gop(struct xen_netbk *netbk,
 	struct gnttab_copy *gop = *gopp;
 	u16 pending_idx = *((u16 *)skb->data);
 	struct skb_shared_info *shinfo = skb_shinfo(skb);
+	struct pending_tx_info *tx_info;
 	int nr_frags = shinfo->nr_frags;
 	int i, err, start;
+	u16 peek; /* peek into next tx request */
 
 	/* Check status of header. */
 	err = gop->status;
@@ -1038,11 +1167,20 @@ static int xen_netbk_tx_check_gop(struct xen_netbk *netbk,
 
 	for (i = start; i < nr_frags; i++) {
 		int j, newerr;
+		pending_ring_idx_t head;
 
 		pending_idx = frag_get_pending_idx(&shinfo->frags[i]);
+		tx_info = &netbk->pending_tx_info[pending_idx];
+		head = tx_info->head;
 
 		/* Check error status: if okay then remember grant handle. */
-		newerr = (++gop)->status;
+		do {
+			newerr = (++gop)->status;
+			if (newerr)
+				break;
+			peek = netbk->pending_ring[pending_index(++head)];
+		} while (!pending_tx_is_head(netbk, peek));
+
 		if (likely(!newerr)) {
 			/* Had a previous error? Invalidate this fragment. */
 			if (unlikely(err))
@@ -1267,11 +1405,12 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 	struct sk_buff *skb;
 	int ret;
 
-	while (((nr_pending_reqs(netbk) + MAX_SKB_FRAGS) < MAX_PENDING_REQS) &&
+	while ((nr_pending_reqs(netbk) + XEN_NETIF_NR_SLOTS_MIN
+		 < MAX_PENDING_REQS) &&
 		!list_empty(&netbk->net_schedule_list)) {
 		struct xenvif *vif;
 		struct xen_netif_tx_request txreq;
-		struct xen_netif_tx_request txfrags[MAX_SKB_FRAGS];
+		struct xen_netif_tx_request txfrags[max_skb_slots];
 		struct page *page;
 		struct xen_netif_extra_info extras[XEN_NETIF_EXTRA_TYPE_MAX-1];
 		u16 pending_idx;
@@ -1332,7 +1471,8 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 				continue;
 		}
 
-		ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do);
+		ret = netbk_count_requests(vif, &txreq, idx,
+					   txfrags, work_to_do);
 		if (unlikely(ret < 0))
 			continue;
 
@@ -1359,7 +1499,7 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 		pending_idx = netbk->pending_ring[index];
 
 		data_len = (txreq.size > PKT_PROT_LEN &&
-			    ret < MAX_SKB_FRAGS) ?
+			    ret < XEN_NETIF_NR_SLOTS_MIN) ?
 			PKT_PROT_LEN : txreq.size;
 
 		skb = alloc_skb(data_len + NET_SKB_PAD + NET_IP_ALIGN,
@@ -1409,6 +1549,7 @@ static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
 		memcpy(&netbk->pending_tx_info[pending_idx].req,
 		       &txreq, sizeof(txreq));
 		netbk->pending_tx_info[pending_idx].vif = vif;
+		netbk->pending_tx_info[pending_idx].head = index;
 		*((u16 *)skb->data) = pending_idx;
 
 		__skb_put(skb, data_len);
@@ -1539,7 +1680,10 @@ static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx,
 {
 	struct xenvif *vif;
 	struct pending_tx_info *pending_tx_info;
-	pending_ring_idx_t index;
+	pending_ring_idx_t head;
+	u16 peek; /* peek into next tx request */
+
+	BUG_ON(netbk->mmap_pages[pending_idx] == (void *)(~0UL));
 
 	/* Already complete? */
 	if (netbk->mmap_pages[pending_idx] == NULL)
@@ -1548,13 +1692,33 @@ static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx,
 	pending_tx_info = &netbk->pending_tx_info[pending_idx];
 
 	vif = pending_tx_info->vif;
+	head = pending_tx_info->head;
 
-	make_tx_response(vif, &pending_tx_info->req, status);
+	BUG_ON(!pending_tx_is_head(netbk, head));
+	BUG_ON(netbk->pending_ring[pending_index(head)] != pending_idx);
 
-	index = pending_index(netbk->pending_prod++);
-	netbk->pending_ring[index] = pending_idx;
+	do {
+		pending_ring_idx_t index;
+		pending_ring_idx_t idx = pending_index(head);
+		u16 info_idx = netbk->pending_ring[idx];
 
-	xenvif_put(vif);
+		pending_tx_info = &netbk->pending_tx_info[info_idx];
+		make_tx_response(vif, &pending_tx_info->req, status);
+
+		/* Setting any number other than
+		 * INVALID_PENDING_RING_IDX indicates this slot is
+		 * starting a new packet / ending a previous packet.
+		 */
+		pending_tx_info->head = 0;
+
+		index = pending_index(netbk->pending_prod++);
+		netbk->pending_ring[index] = netbk->pending_ring[info_idx];
+
+		xenvif_put(vif);
+
+		peek = netbk->pending_ring[pending_index(++head)];
+
+	} while (!pending_tx_is_head(netbk, peek));
 
 	netbk->mmap_pages[pending_idx]->mapping = 0;
 	put_page(netbk->mmap_pages[pending_idx]);
@@ -1613,7 +1777,8 @@ static inline int rx_work_todo(struct xen_netbk *netbk)
 static inline int tx_work_todo(struct xen_netbk *netbk)
 {
 
-	if (((nr_pending_reqs(netbk) + MAX_SKB_FRAGS) < MAX_PENDING_REQS) &&
+	if (((nr_pending_reqs(netbk) + XEN_NETIF_NR_SLOTS_MIN)
+	     < MAX_PENDING_REQS) &&
 			!list_empty(&netbk->net_schedule_list))
 		return 1;
 
@@ -1697,6 +1862,13 @@ static int __init netback_init(void)
 	if (!xen_domain())
 		return -ENODEV;
 
+	if (max_skb_slots < XEN_NETIF_NR_SLOTS_MIN) {
+		printk(KERN_INFO
+		       "xen-netback: max_skb_slots too small (%d), bump it to XEN_NETIF_NR_SLOTS_MIN (%d)\n",
+		       max_skb_slots, XEN_NETIF_NR_SLOTS_MIN);
+		max_skb_slots = XEN_NETIF_NR_SLOTS_MIN;
+	}
+
 	xen_netbk_group_nr = num_online_cpus();
 	xen_netbk = vzalloc(sizeof(struct xen_netbk) * xen_netbk_group_nr);
 	if (!xen_netbk)
diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
index 58fadca..3ef3fe0 100644
--- a/include/xen/interface/io/netif.h
+++ b/include/xen/interface/io/netif.h
@@ -13,6 +13,24 @@
 #include <xen/interface/grant_table.h>
 
 /*
+ * Older implementation of Xen network frontend / backend has an
+ * implicit dependency on the MAX_SKB_FRAGS as the maximum number of
+ * ring slots a skb can use. Netfront / netback may not work as
+ * expected when frontend and backend have different MAX_SKB_FRAGS.
+ *
+ * A better approach is to add mechanism for netfront / netback to
+ * negotiate this value. However we cannot fix all possible
+ * frontends, so we need to define a value which states the minimum
+ * slots backend must support.
+ *
+ * The minimum value derives from older Linux kernel's MAX_SKB_FRAGS
+ * (18), which is proved to work with most frontends. Any new backend
+ * which doesn't negotiate with frontend should expect frontend to
+ * send a valid packet using slots up to this value.
+ */
+#define XEN_NETIF_NR_SLOTS_MIN 18
+
+/*
  * Notifications after enqueuing any type of message should be conditional on
  * the appropriate req_event or rsp_event field in the shared ring.
  * If the client sends notification for rx requests then it should specify
-- 
1.7.10.4

[PATCH V5 7/7] xen-netback: don't disconnect frontend when seeing oversize packet

[Wei Liu]

Some frontend drivers are sending packets > 64 KiB in length. This length
overflows the length field in the first slot making the following slots have
an invalid length.

Turn this error back into a non-fatal error by dropping the packet. To avoid
having the following slots having fatal errors, consume all slots in the
packet.

This does not reopen the security hole in XSA-39 as if the packet as an
invalid number of slots it will still hit fatal error case.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
 drivers/net/xen-netback/netback.c |   22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
index 86d533a..8b25124 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -981,12 +981,22 @@ static int netbk_count_requests(struct xenvif *vif,
 
 		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
 		       sizeof(*txp));
-		if (txp->size > first->size) {
-			netdev_err(vif->dev,
-				   "Invalid tx request, slot size %u > remaining size %u\n",
-				   txp->size, first->size);
-			netbk_fatal_tx_err(vif);
-			return -EIO;
+
+		/* If the guest submitted a frame >= 64 KiB then
+		 * first->size overflowed and following slots will
+		 * appear to be larger than the frame.
+		 *
+		 * This cannot be fatal error as there are buggy
+		 * frontends that do this.
+		 *
+		 * Consume all slots and drop the packet.
+		 */
+		if (!drop_err && txp->size > first->size) {
+			if (net_ratelimit())
+				netdev_dbg(vif->dev,
+					   "Invalid tx request, slot size %u > remaining size %u\n",
+					   txp->size, first->size);
+			drop_err = -EIO;
 		}
 
 		first->size -= txp->size;
-- 
1.7.10.4

Re: [PATCH V5 0/7] Bundle fixes for Xen netfront /netback

[David Miller]

From: Wei Liu <wei.liu2@citrix.com>
Date: Mon, 15 Apr 2013 18:06:14 +0100

> 1-3 have been applied to DaveM's tree. Repost them for the people who don't
> want to cherry-pick.

Please stop reposting these already applied patches, thank you.

Re: [PATCH V5 4/7] xen-netfront: frags -> slots in log message

[Ian Campbell]

On Mon, 2013-04-15 at 18:06 +0100, Wei Liu wrote:
> Also fix a typo in comment.
> 
> Signed-off-by: Wei Liu <wei.liu2@citrix.com>

Acked-by: Ian Campbell <ian.campbell@citrix.com>

Re: [PATCH V5 0/7] Bundle fixes for Xen netfront /netback

[Ian Campbell]

On Mon, 2013-04-15 at 18:38 +0100, David Miller wrote:
> From: Wei Liu <wei.liu2@citrix.com>
> Date: Mon, 15 Apr 2013 18:06:14 +0100
> 
> > 1-3 have been applied to DaveM's tree. Repost them for the people who don't
> > want to cherry-pick.
> 
> Please stop reposting these already applied patches, thank you.

Yes, please, it confuses me too (I keep trying to re-review them!).

Anyway are you happy to take 4..7 as/when I go through and Ack or would
you like Wei to repost them without the already applied precursor?

Ian.

Re: [PATCH V5 5/7] xen-netfront: reduce gso_max_size to account for max TCP header

[Ian Campbell]

On Mon, 2013-04-15 at 18:06 +0100, Wei Liu wrote:
> The maximum packet including header that can be handled by netfront / netback
> wire format is 65535. Reduce gso_max_size accordingly.
> 
> Drop skb and print warning when skb->len > 65535. This can 1) save the effort
> to send malformed packet to netback, 2) help spotting misconfiguration of
> netfront in the future.
> 
> Signed-off-by: Wei Liu <wei.liu2@citrix.com>

Acked-by: Ian Campbell <ian.campbell@citrix.com>

(one commentlet is that you've used 65535 a few times (or left existing
uses) but you've also just defined XEN_NETIF_MAX_TX_SIZE which would be
a better thing to use)

> ---
>  drivers/net/xen-netfront.c       |   16 ++++++++++++++--
>  include/xen/interface/io/netif.h |    1 +
>  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 1bb2e20..7c53d0c 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -36,7 +36,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/ethtool.h>
>  #include <linux/if_ether.h>
> -#include <linux/tcp.h>
> +#include <net/tcp.h>
>  #include <linux/udp.h>
>  #include <linux/moduleparam.h>
>  #include <linux/mm.h>
> @@ -547,6 +547,16 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	unsigned int len = skb_headlen(skb);
>  	unsigned long flags;
>  
> +	/* If skb->len is too big for wire format, drop skb and alert
> +	 * user about misconfiguration.
> +	 */
> +	if (unlikely(skb->len > XEN_NETIF_MAX_TX_SIZE)) {
> +		net_alert_ratelimited(
> +			"xennet: skb->len = %u, too big for wire format\n",
> +			skb->len);
> +		goto drop;
> +	}
> +
>  	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
>  		xennet_count_skb_frag_slots(skb);
>  	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
> @@ -1058,7 +1068,7 @@ err:
>  
>  static int xennet_change_mtu(struct net_device *dev, int mtu)
>  {
> -	int max = xennet_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
> +	int max = xennet_can_sg(dev) ? 65535 - MAX_TCP_HEADER : ETH_DATA_LEN;
>  
>  	if (mtu > max)
>  		return -EINVAL;
> @@ -1362,6 +1372,8 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev)
>  	SET_ETHTOOL_OPS(netdev, &xennet_ethtool_ops);
>  	SET_NETDEV_DEV(netdev, &dev->dev);
>  
> +	netif_set_gso_max_size(netdev, 65535 - MAX_TCP_HEADER);
> +
>  	np->netdev = netdev;
>  
>  	netif_carrier_off(netdev);
> diff --git a/include/xen/interface/io/netif.h b/include/xen/interface/io/netif.h
> index 9dfc120..58fadca 100644
> --- a/include/xen/interface/io/netif.h
> +++ b/include/xen/interface/io/netif.h
> @@ -47,6 +47,7 @@
>  #define _XEN_NETTXF_extra_info		(3)
>  #define  XEN_NETTXF_extra_info		(1U<<_XEN_NETTXF_extra_info)
>  
> +#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
>  struct xen_netif_tx_request {
>      grant_ref_t gref;      /* Reference to buffer page */
>      uint16_t offset;       /* Offset within buffer page */

Re: [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Ian Campbell]

On Mon, 2013-04-15 at 18:06 +0100, Wei Liu wrote:
> This patch tries to coalesce tx requests when constructing grant copy
> structures. It enables netback to deal with situation when frontend's
> MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.
> 
> With the help of coalescing, this patch tries to address two regressions and
> avoid reopening the security hole in XSA-39.
> 
> Regression 1. The reduction of the number of supported ring entries (slots)
> per packet (from 18 to 17). This regression has been around for some time but
> remains unnoticed until XSA-39 security fix. This is fixed by coalescing
> slots.
> 
> Regression 2. The XSA-39 security fix turning "too many frags" errors from
> just dropping the packet to a fatal error and disabling the VIF. This is fixed
> by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17)
> which rules out false positive (using 18 slots is legit) and dropping packets
> using 19 to `max_skb_slots` slots.
> 
> To avoid reopening security hole in XSA-39, frontend sending packet using more
> than max_skb_slots is considered malicious.
> 
> The behavior of netback for packet is thus:
> 
>     1-18            slots: valid
>    19-max_skb_slots slots: drop and respond with an error
>    max_skb_slots+   slots: fatal error
> 
> max_skb_slots is configurable by admin, default value is 20.
> 
> Also change variable name from "frags" to "slots" in netbk_count_requests.
> 
> Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be
> fixed with separate patch.
> 
> Signed-off-by: Wei Liu <wei.liu2@citrix.com>

Acked-by: Ian Campbell <ian.campbell@citrix.com>

Re: [PATCH V5 0/7] Bundle fixes for Xen netfront /netback

[David Miller]

From: Ian Campbell <Ian.Campbell@citrix.com>
Date: Wed, 17 Apr 2013 14:49:44 +0100

> On Mon, 2013-04-15 at 18:38 +0100, David Miller wrote:
>> From: Wei Liu <wei.liu2@citrix.com>
>> Date: Mon, 15 Apr 2013 18:06:14 +0100
>> 
>> > 1-3 have been applied to DaveM's tree. Repost them for the people who don't
>> > want to cherry-pick.
>> 
>> Please stop reposting these already applied patches, thank you.
> 
> Yes, please, it confuses me too (I keep trying to re-review them!).
> 
> Anyway are you happy to take 4..7 as/when I go through and Ack or would
> you like Wei to repost them without the already applied precursor?

I'd like a repost, thanks.

Re: [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[annie li]

On 2013-4-15 13:06, Wei Liu wrote:
>   
>   static int netbk_count_requests(struct xenvif *vif,
>   				struct xen_netif_tx_request *first,
> +				RING_IDX first_idx,
>   				struct xen_netif_tx_request *txp,
>   				int work_to_do)
>   {
>   	RING_IDX cons = vif->tx.req_cons;
> -	int frags = 0;
> +	int slots = 0;
> +	int drop_err = 0;
>   
>   	if (!(first->flags & XEN_NETTXF_more_data))
>   		return 0;
>   
>   	do {
> -		if (frags >= work_to_do) {
> -			netdev_err(vif->dev, "Need more frags\n");
> +		if (slots >= work_to_do) {
> +			netdev_err(vif->dev,
> +				   "Asked for %d slots but exceeds this limit\n",
> +				   work_to_do);
>   			netbk_fatal_tx_err(vif);
>   			return -ENODATA;
>   		}
>   
> -		if (unlikely(frags >= MAX_SKB_FRAGS)) {
> -			netdev_err(vif->dev, "Too many frags\n");
> +		/* This guest is really using too many slots and
> +		 * considered malicious.
> +		 */
> +		if (unlikely(slots >= max_skb_slots)) {
> +			netdev_err(vif->dev,
> +				   "Malicious frontend using %d slots, threshold %u\n",
> +				   slots, max_skb_slots);
>   			netbk_fatal_tx_err(vif);
>   			return -E2BIG;

It is possible that vif is freed when packet size is less than 64K here 
but slots required >= max_skb_slots. Alough max_skb_slots can be 
configured, this kind of packets would be dropped in following if 
(!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN).

>   		}
>   
> -		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
> +		/* Xen network protocol had implicit dependency on
> +		 * MAX_SKB_FRAGS. XEN_NETIF_NR_SLOTS_MIN is set to the
> +		 * historical MAX_SKB_FRAGS value 18 to honor the same
> +		 * behavior as before. Any packet using more than 18
> +		 * slots but less than max_skb_slots slots is dropped
> +		 */
> +		if (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN) {
> +			if (net_ratelimit())
> +				netdev_dbg(vif->dev,
> +					   "Too many slots (%d) exceeding limit (%d), dropping packet\n",
> +					   slots, XEN_NETIF_NR_SLOTS_MIN);
> +			drop_err = -E2BIG;

It is possible to drop packets like above(size < 64K && slot >= 
XEN_NETIF_NR_SLOTS_MIN). I do not know how frequently this kind of 
packets appear, maybe some SKBs with compound page(size < 64K && slot >= 
XEN_NETIF_NR_SLOTS_MIN) are dropped here?


Thanks
Annie

Re: [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Wei Liu]

On Sun, Apr 21, 2013 at 11:06:10PM +0100, annie li wrote:
[...]
> > -		if (unlikely(frags >= MAX_SKB_FRAGS)) {
> > -			netdev_err(vif->dev, "Too many frags\n");
> > +		/* This guest is really using too many slots and
> > +		 * considered malicious.
> > +		 */
> > +		if (unlikely(slots >= max_skb_slots)) {
> > +			netdev_err(vif->dev,
> > +				   "Malicious frontend using %d slots, threshold %u\n",
> > +				   slots, max_skb_slots);
> >   			netbk_fatal_tx_err(vif);
> >   			return -E2BIG;
> 
> It is possible that vif is freed when packet size is less than 64K here 
> but slots required >= max_skb_slots. Alough max_skb_slots can be 
> configured, this kind of packets would be dropped in following if 
> (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN).
> 

Yes, because slots required >= max_skb_slots is considered malicious
behavior. The size of the packet doesn't matter here.

> >   		}
> >   
> > -		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
> > +		/* Xen network protocol had implicit dependency on
> > +		 * MAX_SKB_FRAGS. XEN_NETIF_NR_SLOTS_MIN is set to the
> > +		 * historical MAX_SKB_FRAGS value 18 to honor the same
> > +		 * behavior as before. Any packet using more than 18
> > +		 * slots but less than max_skb_slots slots is dropped
> > +		 */
> > +		if (!drop_err && slots >= XEN_NETIF_NR_SLOTS_MIN) {
> > +			if (net_ratelimit())
> > +				netdev_dbg(vif->dev,
> > +					   "Too many slots (%d) exceeding limit (%d), dropping packet\n",
> > +					   slots, XEN_NETIF_NR_SLOTS_MIN);
> > +			drop_err = -E2BIG;
> 
> It is possible to drop packets like above(size < 64K && slot >= 
> XEN_NETIF_NR_SLOTS_MIN). I do not know how frequently this kind of 
> packets appear, maybe some SKBs with compound page(size < 64K && slot >= 
> XEN_NETIF_NR_SLOTS_MIN) are dropped here?
> 

I don't know either, but this is the default behavior of netback so here
we retain the same behavior here to avoid further regression.


Wei.

Re: [Xen-devel] [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[annie li]

On 2013-4-15 13:06, Wei Liu wrote:
> This patch tries to coalesce tx requests when constructing grant copy
> structures. It enables netback to deal with situation when frontend's
> MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.
>
> With the help of coalescing, this patch tries to address two regressions and
> avoid reopening the security hole in XSA-39.
>
> Regression 1. The reduction of the number of supported ring entries (slots)
> per packet (from 18 to 17). This regression has been around for some time but
> remains unnoticed until XSA-39 security fix. This is fixed by coalescing
> slots.
>
> Regression 2. The XSA-39 security fix turning "too many frags" errors from
> just dropping the packet to a fatal error and disabling the VIF. This is fixed
> by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17)
> which rules out false positive (using 18 slots is legit) and dropping packets
> using 19 to `max_skb_slots` slots.
>
> To avoid reopening security hole in XSA-39, frontend sending packet using more
> than max_skb_slots is considered malicious.
>
> The behavior of netback for packet is thus:
>
>      1-18            slots: valid
>     19-max_skb_slots slots: drop and respond with an error
>     max_skb_slots+   slots: fatal error
>
> max_skb_slots is configurable by admin, default value is 20.
>
> Also change variable name from "frags" to "slots" in netbk_count_requests.
>
> Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be
> fixed with separate patch.

Wei,

The RX path dependency on MAX_SKB_FRAGS in netback should not cause 
packet dropping issue like TX path, RX path only calls netif_stop_queue 
if RX ring is full.
What is your plan on RX path? Is it in your schedule or you started it 
already?

Thanks
Annie

Re: [Xen-devel] [PATCH V5 6/7] xen-netback: coalesce slots in TX path and fix regressions

[Wei Liu]

On Tue, Apr 30, 2013 at 07:06:01PM +0100, annie li wrote:
> 
> On 2013-4-15 13:06, Wei Liu wrote:
> > This patch tries to coalesce tx requests when constructing grant copy
> > structures. It enables netback to deal with situation when frontend's
> > MAX_SKB_FRAGS is larger than backend's MAX_SKB_FRAGS.
> >
> > With the help of coalescing, this patch tries to address two regressions and
> > avoid reopening the security hole in XSA-39.
> >
> > Regression 1. The reduction of the number of supported ring entries (slots)
> > per packet (from 18 to 17). This regression has been around for some time but
> > remains unnoticed until XSA-39 security fix. This is fixed by coalescing
> > slots.
> >
> > Regression 2. The XSA-39 security fix turning "too many frags" errors from
> > just dropping the packet to a fatal error and disabling the VIF. This is fixed
> > by coalescing slots (handling 18 slots when backend's MAX_SKB_FRAGS is 17)
> > which rules out false positive (using 18 slots is legit) and dropping packets
> > using 19 to `max_skb_slots` slots.
> >
> > To avoid reopening security hole in XSA-39, frontend sending packet using more
> > than max_skb_slots is considered malicious.
> >
> > The behavior of netback for packet is thus:
> >
> >      1-18            slots: valid
> >     19-max_skb_slots slots: drop and respond with an error
> >     max_skb_slots+   slots: fatal error
> >
> > max_skb_slots is configurable by admin, default value is 20.
> >
> > Also change variable name from "frags" to "slots" in netbk_count_requests.
> >
> > Please note that RX path still has dependency on MAX_SKB_FRAGS. This will be
> > fixed with separate patch.
> 
> Wei,
> 
> The RX path dependency on MAX_SKB_FRAGS in netback should not cause 
> packet dropping issue like TX path, RX path only calls netif_stop_queue 
> if RX ring is full.
> What is your plan on RX path? Is it in your schedule or you started it 
> already?
> 

Not yet planned or started. My general idea is that we need to remove
all dependency on MAX_SKB_FRAGS.

I'm working on bug fix for libxl now, so I might get to that later.


Wei.

> Thanks
> Annie