[PATCH net 1/2] tun: read vnet_hdr_sz once

[Willem de Bruijn]

From: Willem de Bruijn <will...@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.

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

Signed-off-by: Willem de Bruijn <will...@google.com>
Reported-by: Dmitry Vyukov <dvyu...@google.com>
CC: Eric Dumazet <eduma...@google.com>
---
 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 2cd10b26b650..bfabe180053e 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1170,9 +1170,11 @@ 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;
 
                if (!copy_from_iter_full(&gso, sizeof(gso), from))
                        return -EFAULT;
@@ -1183,7 +1185,7 @@ 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) {
@@ -1335,7 +1337,7 @@ 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;
 
-- 
2.11.0.483.g087da7b7c-goog