[Patch net] kcm: fix a null pointer dereference in kcm_sendmsg()

[Cong Wang]

In commit 98e3862ca2b1 ("kcm: fix 0-length case for kcm_sendmsg()")
I tried to avoid skb allocation for 0-length case, but missed
a check for NULL pointer in the non EOR case.

Fixes: 98e3862ca2b1 ("kcm: fix 0-length case for kcm_sendmsg()")
Reported-by: Dmitry Vyukov <dvyu...@google.com>
Cc: Tom Herbert <t...@herbertland.com>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
---
 net/kcm/kcmsock.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 64f0e85..a646f34 100644
--- a/net/kcm/kcmsock.c
+++ b/net/kcm/kcmsock.c
@@ -1044,8 +1044,10 @@ static int kcm_sendmsg(struct socket *sock, struct 
msghdr *msg, size_t len)
        } else {
                /* Message not complete, save state */
 partial_message:
-               kcm->seq_skb = head;
-               kcm_tx_msg(head)->last_skb = skb;
+               if (head) {
+                       kcm->seq_skb = head;
+                       kcm_tx_msg(head)->last_skb = skb;
+               }
        }
 
        KCM_STATS_ADD(kcm->stats.tx_bytes, copied);
-- 
2.5.5