[PATCH 3.10 263/268] tun: read vnet_hdr_sz once

[Willy Tarreau]

From: Willem de Bruijn <willemb@xxxxxxxxxx>

commit e1edab87faf6ca30cd137e0795bc73aa9a9a22ec 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.

Read this value once and cache locally, as it can be updated between
the test and use (TOCTOU).

[js] we have TUN_VNET_HDR in 3.12

Signed-off-by: Willem de Bruijn <willemb@xxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
CC: Eric Dumazet <edumazet@xxxxxxxxxx>
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
[wt: s/READ_ONCE/ACCESS_ONCE]

Signed-off-by: Willy Tarreau <w@xxxxxx>
---
 drivers/net/tun.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index ea6ada3..7bbc43f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1087,9 +1087,11 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	}
 
 	if (tun->flags & TUN_VNET_HDR) {
-		if (len < tun->vnet_hdr_sz)
+		int vnet_hdr_sz = ACCESS_ONCE(tun->vnet_hdr_sz);
+
+		if (len < vnet_hdr_sz)
 			return -EINVAL;
-		len -= tun->vnet_hdr_sz;
+		len -= vnet_hdr_sz;
 
 		if (memcpy_fromiovecend((void *)&gso, iv, offset, sizeof(gso)))
 			return -EFAULT;
@@ -1100,7 +1102,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 
 		if (gso.hdr_len > len)
 			return -EINVAL;
-		offset += tun->vnet_hdr_sz;
+		offset += vnet_hdr_sz;
 	}
 
 	if ((tun->flags & TUN_TYPE_MASK) == TUN_TAP_DEV) {
@@ -1275,7 +1277,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 	int vnet_hdr_sz = 0;
 
 	if (tun->flags & TUN_VNET_HDR)
-		vnet_hdr_sz = tun->vnet_hdr_sz;
+		vnet_hdr_sz = ACCESS_ONCE(tun->vnet_hdr_sz);
 
 	if (!(tun->flags & TUN_NO_PI)) {
 		if ((len -= sizeof(pi)) < 0)
-- 
2.8.0.rc2.1.gbe9624a