target/linux/generic/backport-5.4/080-wireguard-0088-wireguard-send-account-for-mtu-0-devices.patch

   1 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
   2 From: "Jason A. Donenfeld" <Jason@zx2c4.com>
   3 Date: Fri, 14 Feb 2020 23:57:22 +0100
   4 Subject: [PATCH] wireguard: send: account for mtu=0 devices
   5
   6 commit 175f1ca9a9ed8689d2028da1a7c624bb4fb4ff7e upstream.
   7
   8 It turns out there's an easy way to get packets queued up while still
   9 having an MTU of zero, and that's via persistent keep alive. This commit
  10 makes sure that in whatever condition, we don't wind up dividing by
  11 zero. Note that an MTU of zero for a wireguard interface is something
  12 quasi-valid, so I don't think the correct fix is to limit it via
  13 min_mtu. This can be reproduced easily with:
  14
  15 ip link add wg0 type wireguard
  16 ip link add wg1 type wireguard
  17 ip link set wg0 up mtu 0
  18 ip link set wg1 up
  19 wg set wg0 private-key <(wg genkey)
  20 wg set wg1 listen-port 1 private-key <(wg genkey) peer $(wg show wg0 public-key)
  21 wg set wg0 peer $(wg show wg1 public-key) persistent-keepalive 1 endpoint 127.0.0.1:1
  22
  23 However, while min_mtu=0 seems fine, it makes sense to restrict the
  24 max_mtu. This commit also restricts the maximum MTU to the greatest
  25 number for which rounding up to the padding multiple won't overflow a
  26 signed integer. Packets this large were always rejected anyway
  27 eventually, due to checks deeper in, but it seems more sound not to even
  28 let the administrator configure something that won't work anyway.
  29
  30 We use this opportunity to clean up this function a bit so that it's
  31 clear which paths we're expecting.
  32
  33 Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
  34 Cc: Eric Dumazet <eric.dumazet@gmail.com>
  35 Reviewed-by: Eric Dumazet <edumazet@google.com>
  36 Signed-off-by: David S. Miller <davem@davemloft.net>
  37 Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
  38 ---
  39  drivers/net/wireguard/device.c |  7 ++++---
  40  drivers/net/wireguard/send.c   | 16 +++++++++++-----
  41  2 files changed, 15 insertions(+), 8 deletions(-)
  42
  43 --- a/drivers/net/wireguard/device.c
  44 +++ b/drivers/net/wireguard/device.c
  45 @@ -258,6 +258,8 @@ static void wg_setup(struct net_device *
  46         enum { WG_NETDEV_FEATURES = NETIF_F_HW_CSUM | NETIF_F_RXCSUM |
  47                                     NETIF_F_SG | NETIF_F_GSO |
  48                                     NETIF_F_GSO_SOFTWARE | NETIF_F_HIGHDMA };
  49 +       const int overhead = MESSAGE_MINIMUM_LENGTH + sizeof(struct udphdr) +
  50 +                            max(sizeof(struct ipv6hdr), sizeof(struct iphdr));
  51
  52         dev->netdev_ops = &netdev_ops;
  53         dev->hard_header_len = 0;
  54 @@ -271,9 +273,8 @@ static void wg_setup(struct net_device *
  55         dev->features |= WG_NETDEV_FEATURES;
  56         dev->hw_features |= WG_NETDEV_FEATURES;
  57         dev->hw_enc_features |= WG_NETDEV_FEATURES;
  58 -       dev->mtu = ETH_DATA_LEN - MESSAGE_MINIMUM_LENGTH -
  59 -                  sizeof(struct udphdr) -
  60 -                  max(sizeof(struct ipv6hdr), sizeof(struct iphdr));
  61 +       dev->mtu = ETH_DATA_LEN - overhead;
  62 +       dev->max_mtu = round_down(INT_MAX, MESSAGE_PADDING_MULTIPLE) - overhead;
  63
  64         SET_NETDEV_DEVTYPE(dev, &device_type);
  65
  66 --- a/drivers/net/wireguard/send.c
  67 +++ b/drivers/net/wireguard/send.c
  68 @@ -143,16 +143,22 @@ static void keep_key_fresh(struct wg_pee
  69
  70  static unsigned int calculate_skb_padding(struct sk_buff *skb)
  71  {
  72 +       unsigned int padded_size, last_unit = skb->len;
  73 +
  74 +       if (unlikely(!PACKET_CB(skb)->mtu))
  75 +               return ALIGN(last_unit, MESSAGE_PADDING_MULTIPLE) - last_unit;
  76 +
  77         /* We do this modulo business with the MTU, just in case the networking
  78          * layer gives us a packet that's bigger than the MTU. In that case, we
  79          * wouldn't want the final subtraction to overflow in the case of the
  80 -        * padded_size being clamped.
  81 +        * padded_size being clamped. Fortunately, that's very rarely the case,
  82 +        * so we optimize for that not happening.
  83          */
  84 -       unsigned int last_unit = skb->len % PACKET_CB(skb)->mtu;
  85 -       unsigned int padded_size = ALIGN(last_unit, MESSAGE_PADDING_MULTIPLE);
  86 +       if (unlikely(last_unit > PACKET_CB(skb)->mtu))
  87 +               last_unit %= PACKET_CB(skb)->mtu;
  88
  89 -       if (padded_size > PACKET_CB(skb)->mtu)
  90 -               padded_size = PACKET_CB(skb)->mtu;
  91 +       padded_size = min(PACKET_CB(skb)->mtu,
  92 +                         ALIGN(last_unit, MESSAGE_PADDING_MULTIPLE));
  93         return padded_size - last_unit;
  94  }
  95