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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 111b972e3053eab7ce299721165d8888494a7059..c31d8e74f131ed5da2e7cad82e655096d7126996 100644 (file)
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1108,9 +1108,11 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
        }
 
        if (tun->flags & IFF_VNET_HDR) {
-               if (len < tun->vnet_hdr_sz)
+               int vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
+
+               if (len < vnet_hdr_sz)
                        return -EINVAL;
-               len -= tun->vnet_hdr_sz;
+               len -= vnet_hdr_sz;
 
                n = copy_from_iter(&gso, sizeof(gso), from);
                if (n != sizeof(gso))
@@ -1122,7 +1124,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 
                if (tun16_to_cpu(tun, gso.hdr_len) > len)
                        return -EINVAL;
-               iov_iter_advance(from, tun->vnet_hdr_sz - sizeof(gso));
+               iov_iter_advance(from, vnet_hdr_sz - sizeof(gso));
        }
 
        if ((tun->flags & TUN_TYPE_MASK) == IFF_TAP) {
@@ -1301,7 +1303,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
                vlan_hlen = VLAN_HLEN;
 
        if (tun->flags & IFF_VNET_HDR)
-               vnet_hdr_sz = tun->vnet_hdr_sz;
+               vnet_hdr_sz = READ_ONCE(tun->vnet_hdr_sz);
 
        total = skb->len + vlan_hlen + vnet_hdr_sz;