[PATCH 5.1 30/98] net: correct udp zerocopy refcnt also when zerocopy only on append

[Greg Kroah-Hartman]

From: Willem de Bruijn <will...@google.com>

[ Upstream commit 522924b583082f51b8a2406624a2f27c22119b20 ]

The below patch fixes an incorrect zerocopy refcnt increment when
appending with MSG_MORE to an existing zerocopy udp skb.

  send(.., MSG_ZEROCOPY | MSG_MORE);    // refcnt 1
  send(.., MSG_ZEROCOPY | MSG_MORE);    // refcnt still 1 (bar frags)

But it missed that zerocopy need not be passed at the first send. The
right test whether the uarg is newly allocated and thus has extra
refcnt 1 is not !skb, but !skb_zcopy.

  send(.., MSG_MORE);                   // <no uarg>
  send(.., MSG_ZEROCOPY);               // refcnt 1

Fixes: 100f6d8e09905 ("net: correct zerocopy refcnt with udp MSG_MORE")
Reported-by: syzbot <syzkal...@googlegroups.com>
Signed-off-by: Willem de Bruijn <will...@google.com>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
 net/ipv4/ip_output.c  |    2 +-
 net/ipv6/ip6_output.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -923,7 +923,7 @@ static int __ip_append_data(struct sock
                uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
                if (!uarg)
                        return -ENOBUFS;
-               extra_uref = !skb;      /* only extra ref if !MSG_MORE */
+               extra_uref = !skb_zcopy(skb);   /* only ref on new uarg */
                if (rt->dst.dev->features & NETIF_F_SG &&
                    csummode == CHECKSUM_PARTIAL) {
                        paged = true;
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1344,7 +1344,7 @@ emsgsize:
                uarg = sock_zerocopy_realloc(sk, length, skb_zcopy(skb));
                if (!uarg)
                        return -ENOBUFS;
-               extra_uref = !skb;      /* only extra ref if !MSG_MORE */
+               extra_uref = !skb_zcopy(skb);   /* only ref on new uarg */
                if (rt->dst.dev->features & NETIF_F_SG &&
                    csummode == CHECKSUM_PARTIAL) {
                        paged = true;