udp: consistently apply ufo or fragmentation

When iteratively building a UDP datagram with MSG_MORE and that
datagram exceeds MTU, consistently choose UFO or fragmentation.

Once skb_is_gso, always apply ufo. Conversely, once a datagram is
split across multiple skbs, do not consider ufo.

Sendpage already maintains the first invariant, only add the second.
IPv6 does not have a sendpage implementation to modify.

A gso skb must have a partial checksum, do not follow sk_no_check_tx
in udp_send_skb.

Found by syzkaller.

Fixes: e89e9cf539a2 ("[IPv4/IPv6]: UFO Scatter-gather approach")
Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

3 files changed, 10 insertions, 7 deletions

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 50c74cd890bc..e153c40c2436 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -965,11 +965,12 @@ static int __ip_append_data(struct sock *sk,
                 csummode = CHECKSUM_PARTIAL;
 
         cork->length += length;
-        if ((((length + (skb ? skb->len : fragheaderlen)) > mtu) ||
-             (skb && skb_is_gso(skb))) &&
+        if ((skb && skb_is_gso(skb)) ||
+            (((length + (skb ? skb->len : fragheaderlen)) > mtu) &&
+            (skb_queue_len(queue) <= 1) &&
             (sk->sk_protocol == IPPROTO_UDP) &&
             (rt->dst.dev->features & NETIF_F_UFO) && !dst_xfrm(&rt->dst) &&
-            (sk->sk_type == SOCK_DGRAM) && !sk->sk_no_check_tx) {
+            (sk->sk_type == SOCK_DGRAM) && !sk->sk_no_check_tx)) {
                 err = ip_ufo_append_data(sk, queue, getfrag, from, length,
                                          hh_len, fragheaderlen, transhdrlen,
                                          maxfraglen, flags);
@@ -1288,6 +1289,7 @@ ssize_t	ip_append_page(struct sock *sk, struct flowi4 *fl4, struct page *page,
                 return -EINVAL;
 
         if ((size + skb->len > mtu) &&
+            (skb_queue_len(&sk->sk_write_queue) == 1) &&
             (sk->sk_protocol == IPPROTO_UDP) &&
             (rt->dst.dev->features & NETIF_F_UFO)) {
                 if (skb->ip_summed != CHECKSUM_PARTIAL)
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index e6276fa3750b..a7c804f73990 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -802,7 +802,7 @@ static int udp_send_skb(struct sk_buff *skb, struct flowi4 *fl4)
         if (is_udplite)                                  /*     UDP-Lite      */
                 csum = udplite_csum(skb);
 
-        else if (sk->sk_no_check_tx) {   /* UDP csum disabled */
+        else if (sk->sk_no_check_tx && !skb_is_gso(skb)) {   /* UDP csum off */
 
                 skb->ip_summed = CHECKSUM_NONE;
                 goto send;
diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 162efba0d0cd..2dfe50d8d609 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1381,11 +1381,12 @@ emsgsize:
          */
 
         cork->length += length;
-        if ((((length + (skb ? skb->len : headersize)) > mtu) ||
-             (skb && skb_is_gso(skb))) &&
+        if ((skb && skb_is_gso(skb)) ||
+            (((length + (skb ? skb->len : headersize)) > mtu) &&
+            (skb_queue_len(queue) <= 1) &&
             (sk->sk_protocol == IPPROTO_UDP) &&
             (rt->dst.dev->features & NETIF_F_UFO) && !dst_xfrm(&rt->dst) &&
-            (sk->sk_type == SOCK_DGRAM) && !udp_get_no_check6_tx(sk)) {
+            (sk->sk_type == SOCK_DGRAM) && !udp_get_no_check6_tx(sk))) {
                 err = ip6_ufo_append_data(sk, queue, getfrag, from, length,
                                           hh_len, fragheaderlen, exthdrlen,
                                           transhdrlen, mtu, flags, fl6);