main/linux-grsec: security fix (CVE-2015-8550, xsa-155). Fixes #5159

8 files changed, 466 insertions, 1 deletions

diff --git a/main/linux-grsec/APKBUILD b/main/linux-grsec/APKBUILD
index 475fce0d63..c43bda0cab 100644
--- a/main/linux-grsec/APKBUILD
+++ b/main/linux-grsec/APKBUILD
@@ -7,7 +7,7 @@ case $pkgver in
 *.*.*)	_kernver=${pkgver%.*};;
 *.*)	_kernver=${pkgver};;
 esac
-pkgrel=0
+pkgrel=1
 pkgdesc="Linux kernel with grsecurity"
 url=http://grsecurity.net
 depends="mkinitfs"
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0001-xen-Add-RING_COPY_REQUEST.patch b/main/linux-grsec/xsa155-linux-xsa155-0001-xen-Add-RING_COPY_REQUEST.patch
new file mode 100644
index 0000000000..5496a09821
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0001-xen-Add-RING_COPY_REQUEST.patch
@@ -0,0 +1,57 @@
+From 4e2bc423e0cef0a42f93d989c0980301df1bd462 Mon Sep 17 00:00:00 2001
+From: David Vrabel <david.vrabel@citrix.com>
+Date: Fri, 30 Oct 2015 14:58:08 +0000
+Subject: [PATCH 1/7] xen: Add RING_COPY_REQUEST()
+
+Using RING_GET_REQUEST() on a shared ring is easy to use incorrectly
+(i.e., by not considering that the other end may alter the data in the
+shared ring while it is being inspected).  Safe usage of a request
+generally requires taking a local copy.
+
+Provide a RING_COPY_REQUEST() macro to use instead of
+RING_GET_REQUEST() and an open-coded memcpy().  This takes care of
+ensuring that the copy is done correctly regardless of any possible
+compiler optimizations.
+
+Use a volatile source to prevent the compiler from reordering or
+omitting the copy.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+v2: Update about GCC and bitfields.
+---
+ include/xen/interface/io/ring.h | 14 ++++++++++++++
+ 1 file changed, 14 insertions(+)
+
+diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
+index 7d28aff..7dc685b 100644
+--- a/include/xen/interface/io/ring.h
++++ b/include/xen/interface/io/ring.h
+@@ -181,6 +181,20 @@ struct __name##_back_ring {						\
+ #define RING_GET_REQUEST(_r, _idx)					\
+     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
+ 
++/*
++ * Get a local copy of a request.
++ *
++ * Use this in preference to RING_GET_REQUEST() so all processing is
++ * done on a local copy that cannot be modified by the other end.
++ *
++ * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause this
++ * to be ineffective where _req is a struct which consists of only bitfields.
++ */
++#define RING_COPY_REQUEST(_r, _idx, _req) do {				\
++	/* Use volatile to force the copy into _req. */			\
++	*(_req) = *(volatile typeof(_req))RING_GET_REQUEST(_r, _idx);	\
++} while (0)
++
+ #define RING_GET_RESPONSE(_r, _idx)					\
+     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
+ 
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0002-xen-netback-don-t-use-last-request-to-determine-mini.patch b/main/linux-grsec/xsa155-linux-xsa155-0002-xen-netback-don-t-use-last-request-to-determine-mini.patch
new file mode 100644
index 0000000000..d60f4d0e39
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0002-xen-netback-don-t-use-last-request-to-determine-mini.patch
@@ -0,0 +1,40 @@
+From 100ac372a0e07ccc8c508c3884fa9020cfe08094 Mon Sep 17 00:00:00 2001
+From: David Vrabel <david.vrabel@citrix.com>
+Date: Fri, 30 Oct 2015 15:16:01 +0000
+Subject: [PATCH 2/7] xen-netback: don't use last request to determine minimum
+ Tx credit
+
+The last from guest transmitted request gives no indication about the
+minimum amount of credit that the guest might need to send a packet
+since the last packet might have been a small one.
+
+Instead allow for the worst case 128 KiB packet.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Reviewed-by: Wei Liu <wei.liu2@citrix.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ drivers/net/xen-netback/netback.c | 4 +---
+ 1 file changed, 1 insertion(+), 3 deletions(-)
+
+diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
+index e481f37..b683581 100644
+--- a/drivers/net/xen-netback/netback.c
++++ b/drivers/net/xen-netback/netback.c
+@@ -679,9 +679,7 @@ static void tx_add_credit(struct xenvif_queue *queue)
+ 	 * Allow a burst big enough to transmit a jumbo packet of up to 128kB.
+ 	 * Otherwise the interface can seize up due to insufficient credit.
+ 	 */
+-	max_burst = RING_GET_REQUEST(&queue->tx, queue->tx.req_cons)->size;
+-	max_burst = min(max_burst, 131072UL);
+-	max_burst = max(max_burst, queue->credit_bytes);
++	max_burst = max(131072UL, queue->credit_bytes);
+ 
+ 	/* Take care that adding a new chunk of credit doesn't wrap to zero. */
+ 	max_credit = queue->remaining_credit + queue->credit_bytes;
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0003-xen-netback-use-RING_COPY_REQUEST-throughout.patch b/main/linux-grsec/xsa155-linux-xsa155-0003-xen-netback-use-RING_COPY_REQUEST-throughout.patch
new file mode 100644
index 0000000000..a6bb03761e
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0003-xen-netback-use-RING_COPY_REQUEST-throughout.patch
@@ -0,0 +1,131 @@
+From 4127e9ccae0eda622421d21132846abdf74f66ed Mon Sep 17 00:00:00 2001
+From: David Vrabel <david.vrabel@citrix.com>
+Date: Fri, 30 Oct 2015 15:17:06 +0000
+Subject: [PATCH 3/7] xen-netback: use RING_COPY_REQUEST() throughout
+
+Instead of open-coding memcpy()s and directly accessing Tx and Rx
+requests, use the new RING_COPY_REQUEST() that ensures the local copy
+is correct.
+
+This is more than is strictly necessary for guest Rx requests since
+only the id and gref fields are used and it is harmless if the
+frontend modifies these.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Reviewed-by: Wei Liu <wei.liu2@citrix.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ drivers/net/xen-netback/netback.c | 30 ++++++++++++++----------------
+ 1 file changed, 14 insertions(+), 16 deletions(-)
+
+diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
+index b683581..1049c34 100644
+--- a/drivers/net/xen-netback/netback.c
++++ b/drivers/net/xen-netback/netback.c
+@@ -258,18 +258,18 @@ static struct xenvif_rx_meta *get_next_rx_buffer(struct xenvif_queue *queue,
+ 						 struct netrx_pending_operations *npo)
+ {
+ 	struct xenvif_rx_meta *meta;
+-	struct xen_netif_rx_request *req;
++	struct xen_netif_rx_request req;
+ 
+-	req = RING_GET_REQUEST(&queue->rx, queue->rx.req_cons++);
++	RING_COPY_REQUEST(&queue->rx, queue->rx.req_cons++, &req);
+ 
+ 	meta = npo->meta + npo->meta_prod++;
+ 	meta->gso_type = XEN_NETIF_GSO_TYPE_NONE;
+ 	meta->gso_size = 0;
+ 	meta->size = 0;
+-	meta->id = req->id;
++	meta->id = req.id;
+ 
+ 	npo->copy_off = 0;
+-	npo->copy_gref = req->gref;
++	npo->copy_gref = req.gref;
+ 
+ 	return meta;
+ }
+@@ -424,7 +424,7 @@ static int xenvif_gop_skb(struct sk_buff *skb,
+ 	struct xenvif *vif = netdev_priv(skb->dev);
+ 	int nr_frags = skb_shinfo(skb)->nr_frags;
+ 	int i;
+-	struct xen_netif_rx_request *req;
++	struct xen_netif_rx_request req;
+ 	struct xenvif_rx_meta *meta;
+ 	unsigned char *data;
+ 	int head = 1;
+@@ -443,15 +443,15 @@ static int xenvif_gop_skb(struct sk_buff *skb,
+ 
+ 	/* Set up a GSO prefix descriptor, if necessary */
+ 	if ((1 << gso_type) & vif->gso_prefix_mask) {
+-		req = RING_GET_REQUEST(&queue->rx, queue->rx.req_cons++);
++		RING_COPY_REQUEST(&queue->rx, queue->rx.req_cons++, &req);
+ 		meta = npo->meta + npo->meta_prod++;
+ 		meta->gso_type = gso_type;
+ 		meta->gso_size = skb_shinfo(skb)->gso_size;
+ 		meta->size = 0;
+-		meta->id = req->id;
++		meta->id = req.id;
+ 	}
+ 
+-	req = RING_GET_REQUEST(&queue->rx, queue->rx.req_cons++);
++	RING_COPY_REQUEST(&queue->rx, queue->rx.req_cons++, &req);
+ 	meta = npo->meta + npo->meta_prod++;
+ 
+ 	if ((1 << gso_type) & vif->gso_mask) {
+@@ -463,9 +463,9 @@ static int xenvif_gop_skb(struct sk_buff *skb,
+ 	}
+ 
+ 	meta->size = 0;
+-	meta->id = req->id;
++	meta->id = req.id;
+ 	npo->copy_off = 0;
+-	npo->copy_gref = req->gref;
++	npo->copy_gref = req.gref;
+ 
+ 	data = skb->data;
+ 	while (data < skb_tail_pointer(skb)) {
+@@ -709,7 +709,7 @@ static void xenvif_tx_err(struct xenvif_queue *queue,
+ 		spin_unlock_irqrestore(&queue->response_lock, flags);
+ 		if (cons == end)
+ 			break;
+-		txp = RING_GET_REQUEST(&queue->tx, cons++);
++		RING_COPY_REQUEST(&queue->tx, cons++, txp);
+ 	} while (1);
+ 	queue->tx.req_cons = cons;
+ }
+@@ -776,8 +776,7 @@ static int xenvif_count_requests(struct xenvif_queue *queue,
+ 		if (drop_err)
+ 			txp = &dropped_tx;
+ 
+-		memcpy(txp, RING_GET_REQUEST(&queue->tx, cons + slots),
+-		       sizeof(*txp));
++		RING_COPY_REQUEST(&queue->tx, cons + slots, txp);
+ 
+ 		/* If the guest submitted a frame >= 64 KiB then
+ 		 * first->size overflowed and following slots will
+@@ -1110,8 +1109,7 @@ static int xenvif_get_extras(struct xenvif_queue *queue,
+ 			return -EBADR;
+ 		}
+ 
+-		memcpy(&extra, RING_GET_REQUEST(&queue->tx, cons),
+-		       sizeof(extra));
++		RING_COPY_REQUEST(&queue->tx, cons, &extra);
+ 		if (unlikely(!extra.type ||
+ 			     extra.type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
+ 			queue->tx.req_cons = ++cons;
+@@ -1320,7 +1318,7 @@ static void xenvif_tx_build_gops(struct xenvif_queue *queue,
+ 
+ 		idx = queue->tx.req_cons;
+ 		rmb(); /* Ensure that we see the request before we copy it. */
+-		memcpy(&txreq, RING_GET_REQUEST(&queue->tx, idx), sizeof(txreq));
++		RING_COPY_REQUEST(&queue->tx, idx, &txreq);
+ 
+ 		/* Credit-based scheduling. */
+ 		if (txreq.size > queue->remaining_credit &&
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0004-xen-blkback-only-read-request-operation-from-shared-.patch b/main/linux-grsec/xsa155-linux-xsa155-0004-xen-blkback-only-read-request-operation-from-shared-.patch
new file mode 100644
index 0000000000..e0e9577c77
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0004-xen-blkback-only-read-request-operation-from-shared-.patch
@@ -0,0 +1,54 @@
+From 084b8c2e77f1ac07e4a3a121ff957c49a9379385 Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
+Date: Tue, 3 Nov 2015 16:34:09 +0000
+Subject: [PATCH 4/7] xen-blkback: only read request operation from shared ring
+ once
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+A compiler may load a switch statement value multiple times, which could
+be bad when the value is in memory shared with the frontend.
+
+When converting a non-native request to a native one, ensure that
+src->operation is only loaded once by using READ_ONCE().
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ drivers/block/xen-blkback/common.h | 8 ++++----
+ 1 file changed, 4 insertions(+), 4 deletions(-)
+
+diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
+index 68e87a0..c929ae2 100644
+--- a/drivers/block/xen-blkback/common.h
++++ b/drivers/block/xen-blkback/common.h
+@@ -408,8 +408,8 @@ static inline void blkif_get_x86_32_req(struct blkif_request *dst,
+ 					struct blkif_x86_32_request *src)
+ {
+ 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j;
+-	dst->operation = src->operation;
+-	switch (src->operation) {
++	dst->operation = READ_ONCE(src->operation);
++	switch (dst->operation) {
+ 	case BLKIF_OP_READ:
+ 	case BLKIF_OP_WRITE:
+ 	case BLKIF_OP_WRITE_BARRIER:
+@@ -456,8 +456,8 @@ static inline void blkif_get_x86_64_req(struct blkif_request *dst,
+ 					struct blkif_x86_64_request *src)
+ {
+ 	int i, n = BLKIF_MAX_SEGMENTS_PER_REQUEST, j;
+-	dst->operation = src->operation;
+-	switch (src->operation) {
++	dst->operation = READ_ONCE(src->operation);
++	switch (dst->operation) {
+ 	case BLKIF_OP_READ:
+ 	case BLKIF_OP_WRITE:
+ 	case BLKIF_OP_WRITE_BARRIER:
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0006-xen-scsiback-safely-copy-requests.patch b/main/linux-grsec/xsa155-linux-xsa155-0006-xen-scsiback-safely-copy-requests.patch
new file mode 100644
index 0000000000..28c6d585b3
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0006-xen-scsiback-safely-copy-requests.patch
@@ -0,0 +1,37 @@
+From 89739c14c72e5c1626a5cd5e09cbb2efeaadb6d8 Mon Sep 17 00:00:00 2001
+From: David Vrabel <david.vrabel@citrix.com>
+Date: Mon, 16 Nov 2015 18:02:32 +0000
+Subject: [PATCH 6/7] xen-scsiback: safely copy requests
+
+The copy of the ring request was lacking a following barrier(),
+potentially allowing the compiler to optimize the copy away.
+
+Use RING_COPY_REQUEST() to ensure the request is copied to local
+memory.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Reviewed-by: Juergen Gross <jgross@suse.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ drivers/xen/xen-scsiback.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/drivers/xen/xen-scsiback.c b/drivers/xen/xen-scsiback.c
+index 43bcae8..ad4eb10 100644
+--- a/drivers/xen/xen-scsiback.c
++++ b/drivers/xen/xen-scsiback.c
+@@ -726,7 +726,7 @@ static int scsiback_do_cmd_fn(struct vscsibk_info *info)
+ 		if (!pending_req)
+ 			return 1;
+ 
+-		ring_req = *RING_GET_REQUEST(ring, rc);
++		RING_COPY_REQUEST(ring, rc, &ring_req);
+ 		ring->req_cons = ++rc;
+ 
+ 		err = prepare_pending_reqs(info, &ring_req, pending_req);
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux-xsa155-0007-xen-pciback-Save-xen_pci_op-commands-before-processi.patch b/main/linux-grsec/xsa155-linux-xsa155-0007-xen-pciback-Save-xen_pci_op-commands-before-processi.patch
new file mode 100644
index 0000000000..2448b94bcf
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux-xsa155-0007-xen-pciback-Save-xen_pci_op-commands-before-processi.patch
@@ -0,0 +1,81 @@
+From f6f4388c917ce96b075a239a4535b8efc6064d14 Mon Sep 17 00:00:00 2001
+From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Date: Mon, 16 Nov 2015 12:40:48 -0500
+Subject: [PATCH 7/7] xen/pciback: Save xen_pci_op commands before processing
+ it
+
+Double fetch vulnerabilities that happen when a variable is
+fetched twice from shared memory but a security check is only
+performed the first time.
+
+The xen_pcibk_do_op function performs a switch statements on the op->cmd
+value which is stored in shared memory. Interestingly this can result
+in a double fetch vulnerability depending on the performed compiler
+optimization.
+
+This patch fixes it by saving the xen_pci_op command before
+processing it. We also use 'barrier' to make sure that the
+compiler does not perform any optimization.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Signed-off-by: Jan Beulich <JBeulich@suse.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+---
+ drivers/xen/xen-pciback/pciback.h     |  1 +
+ drivers/xen/xen-pciback/pciback_ops.c | 15 ++++++++++++++-
+ 2 files changed, 15 insertions(+), 1 deletion(-)
+
+diff --git a/drivers/xen/xen-pciback/pciback.h b/drivers/xen/xen-pciback/pciback.h
+index 58e38d5..4d529f3 100644
+--- a/drivers/xen/xen-pciback/pciback.h
++++ b/drivers/xen/xen-pciback/pciback.h
+@@ -37,6 +37,7 @@ struct xen_pcibk_device {
+ 	struct xen_pci_sharedinfo *sh_info;
+ 	unsigned long flags;
+ 	struct work_struct op_work;
++	struct xen_pci_op op;
+ };
+ 
+ struct xen_pcibk_dev_data {
+diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
+index c4a0666..a0e0e3e 100644
+--- a/drivers/xen/xen-pciback/pciback_ops.c
++++ b/drivers/xen/xen-pciback/pciback_ops.c
+@@ -298,9 +298,11 @@ void xen_pcibk_do_op(struct work_struct *data)
+ 		container_of(data, struct xen_pcibk_device, op_work);
+ 	struct pci_dev *dev;
+ 	struct xen_pcibk_dev_data *dev_data = NULL;
+-	struct xen_pci_op *op = &pdev->sh_info->op;
++	struct xen_pci_op *op = &pdev->op;
+ 	int test_intx = 0;
+ 
++	*op = pdev->sh_info->op;
++	barrier();
+ 	dev = xen_pcibk_get_pci_dev(pdev, op->domain, op->bus, op->devfn);
+ 
+ 	if (dev == NULL)
+@@ -342,6 +344,17 @@ void xen_pcibk_do_op(struct work_struct *data)
+ 		if ((dev_data->enable_intx != test_intx))
+ 			xen_pcibk_control_isr(dev, 0 /* no reset */);
+ 	}
++	pdev->sh_info->op.err = op->err;
++	pdev->sh_info->op.value = op->value;
++#ifdef CONFIG_PCI_MSI
++	if (op->cmd == XEN_PCI_OP_enable_msix && op->err == 0) {
++		unsigned int i;
++
++		for (i = 0; i < op->value; i++)
++			pdev->sh_info->op.msix_entries[i].vector =
++				op->msix_entries[i].vector;
++	}
++#endif
+ 	/* Tell the driver domain that we're done. */
+ 	wmb();
+ 	clear_bit(_XEN_PCIF_active, (unsigned long *)&pdev->sh_info->flags);
+-- 
+2.1.0
+
diff --git a/main/linux-grsec/xsa155-linux43-0005-xen-blkback-read-from-indirect-descriptors-only-once.patch b/main/linux-grsec/xsa155-linux43-0005-xen-blkback-read-from-indirect-descriptors-only-once.patch
new file mode 100644
index 0000000000..6598e40882
--- /dev/null
+++ b/main/linux-grsec/xsa155-linux43-0005-xen-blkback-read-from-indirect-descriptors-only-once.patch
@@ -0,0 +1,65 @@
+From d52f00960c1070c683809faddd35a2223e2b8a6e Mon Sep 17 00:00:00 2001
+From: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
+Date: Tue, 3 Nov 2015 16:40:43 +0000
+Subject: [PATCH 6/7] xen-blkback: read from indirect descriptors only once
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Since indirect descriptors are in memory shared with the frontend, the
+frontend could alter the first_sect and last_sect values after they have
+been validated but before they are recorded in the request.  This may
+result in I/O requests that overflow the foreign page, possibly
+overwriting local pages when the I/O request is executed.
+
+When parsing indirect descriptors, only read first_sect and last_sect
+once.
+
+This is part of XSA155.
+
+CC: stable@vger.kernel.org
+Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
+Signed-off-by: David Vrabel <david.vrabel@citrix.com>
+Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+----
+v2: This is against v4.3
+---
+ drivers/block/xen-blkback/blkback.c | 12 +++++++-----
+ 1 file changed, 7 insertions(+), 5 deletions(-)
+
+diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
+index 6a685ae..f2e7a38 100644
+--- a/drivers/block/xen-blkback/blkback.c
++++ b/drivers/block/xen-blkback/blkback.c
+@@ -950,6 +950,8 @@ static int xen_blkbk_parse_indirect(struct blkif_request *req,
+ 		goto unmap;
+ 
+ 	for (n = 0, i = 0; n < nseg; n++) {
++		uint8_t first_sect, last_sect;
++
+ 		if ((n % SEGS_PER_INDIRECT_FRAME) == 0) {
+ 			/* Map indirect segments */
+ 			if (segments)
+@@ -958,14 +960,14 @@ static int xen_blkbk_parse_indirect(struct blkif_request *req,
+ 		}
+ 		i = n % SEGS_PER_INDIRECT_FRAME;
+ 		pending_req->segments[n]->gref = segments[i].gref;
+-		seg[n].nsec = segments[i].last_sect -
+-			segments[i].first_sect + 1;
+-		seg[n].offset = (segments[i].first_sect << 9);
+-		if ((segments[i].last_sect >= (PAGE_SIZE >> 9)) ||
+-		    (segments[i].last_sect < segments[i].first_sect)) {
++		first_sect = READ_ONCE(segments[i].first_sect);
++		last_sect = READ_ONCE(segments[i].last_sect);
++		if (last_sect >= (PAGE_SIZE >> 9) || last_sect < first_sect) {
+ 			rc = -EINVAL;
+ 			goto unmap;
+ 		}
++		seg[n].nsec = last_sect - first_sect + 1;
++		seg[n].offset = first_sect << 9;
+ 		preq->nr_sects += seg[n].nsec;
+ 	}
+ 
+-- 
+2.1.0
+