tun: read vnet_hdr_sz once

[ 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 3.2:
 - Use ACCESS_ONCE() instead of READ_ONCE()
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 995fd59..9c2e91c 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -625,9 +625,11 @@ static ssize_t tun_get_user(struct tun_struct *tun,
        }
 
        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;
@@ -638,7 +640,7 @@ static ssize_t tun_get_user(struct tun_struct *tun,
 
                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) {
@@ -770,7 +772,7 @@ 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)