[net,2/2] macvtap: read vnet_hdr_size once
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Message ID 20170203232049.38982-3-willemdebruijn.kernel@gmail.com
State Accepted, archived
Delegated to: David Miller
Headers show

Commit Message

Willem de Bruijn Feb. 3, 2017, 11:20 p.m. UTC
From: Willem de Bruijn <willemb@google.com>

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@google.com>
Suggested-by: Eric Dumazet <edumazet@google.com>
---
 drivers/net/macvtap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Eric Dumazet Feb. 3, 2017, 11:27 p.m. UTC | #1
On Fri, 2017-02-03 at 18:20 -0500, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.com>
> 
> 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@google.com>
> Suggested-by: Eric Dumazet <edumazet@google.com>
> ---

Acked-by: Eric Dumazet <edumazet@google.com>

Patch
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diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
index 402618565838..c27011bbe30c 100644
--- a/drivers/net/macvtap.c
+++ b/drivers/net/macvtap.c
@@ -681,7 +681,7 @@  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 = READ_ONCE(q->vnet_hdr_sz);
 
 		err = -EINVAL;
 		if (len < vnet_hdr_len)
@@ -820,7 +820,7 @@  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 = READ_ONCE(q->vnet_hdr_sz);
 		if (iov_iter_count(iter) < vnet_hdr_len)
 			return -EINVAL;