[PATCH 3.16 358/370] tun: read vnet_hdr_sz once

Ben Hutchings
2017-03-10T12:30:56Z

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

[ Upstream commit e1edab87faf6ca30cd137e0795bc73aa9a9a22ec ]

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).

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
CC: Eric Dumazet <edumazet@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16:
- Use ACCESS_ONCE() instead of READ_ONCE()
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/net/tun.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1037,9 +1037,11 @@ static ssize_t tun_get_user(struct tun_s
}

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;
@@ -1050,7 +1052,7 @@ static ssize_t tun_get_user(struct tun_s

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) {
@@ -1233,7 +1235,7 @@ static ssize_t tun_put_user(struct tun_s
vlan_hlen = VLAN_HLEN;

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)