linux-kernel - [PATCH 3.10 144/268] macvtap: read vnet_hdr

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Date:   Mon, 19 Jun 2017 20:30:43 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Willem de Bruijn <willemb@...gle.com>,
        "David S . Miller" <davem@...emloft.net>, Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 144/268] macvtap: read vnet_hdr_size once

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

commit 837585a5375c38d40361cfe64e6fd11e1addb936 upstream.

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>
[wt: s/READ_ONCE/ACCESS_ONCE]

Signed-off-by: Willy Tarreau <w@....eu>
---
 drivers/net/macvtap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 8fc46fc..1c51abb 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -678,7 +678,7 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *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)
@@ -809,7 +809,7 @@ static ssize_t macvtap_put_user(struct macvtap_queue *q,

 	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;

-- 
2.8.0.rc2.1.gbe9624a