linux-kernel - [PATCH 3.16 359/370] macvtap: read vnet_hdr

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Message-ID: <lsq.1489146383.718762664@decadent.org.uk>
Date:   Fri, 10 Mar 2017 11:46:23 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Willem de Bruijn" <willemb@...gle.com>,
        "Eric Dumazet" <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 3.16 359/370] macvtap: read vnet_hdr_size once

3.16.42-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Willem de Bruijn <willemb@...gle.com>

[ Upstream commit 837585a5375c38d40361cfe64e6fd11e1addb936 ]

When IFF_VNET_HDR is enabled, a virtio_net header must precede data.
Data length is verified to be greater than or equal to expected header
length tun->vnet_hdr_sz before copying.

Macvtap functions read the value once, but unless READ_ONCE is used,
the compiler may ignore this and read multiple times. Enforce a single
read and locally cached value to avoid updates between test and use.

Signed-off-by: Willem de Bruijn <willemb@...gle.com>
Suggested-by: Eric Dumazet <edumazet@...gle.com>
Acked-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
[bwh: BAckported to 3.16:
 - Use ACCESS_ONCE() instead of READ_ONCE()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/net/macvtap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -657,7 +657,7 @@ static ssize_t macvtap_get_user(struct m
 	size_t linear;
 
 	if (q->flags & IFF_VNET_HDR) {
-		vnet_hdr_len = q->vnet_hdr_sz;
+		vnet_hdr_len = ACCESS_ONCE(q->vnet_hdr_sz);
 
 		err = -EINVAL;
 		if (len < vnet_hdr_len)
@@ -793,7 +793,7 @@ static ssize_t macvtap_put_user(struct m
 
 	if (q->flags & IFF_VNET_HDR) {
 		struct virtio_net_hdr vnet_hdr;
-		vnet_hdr_len = q->vnet_hdr_sz;
+		vnet_hdr_len = ACCESS_ONCE(q->vnet_hdr_sz);
 		if ((len -= vnet_hdr_len) < 0)
 			return -EINVAL;