net: compat_sys_recvmmsg user timespec arg can be NULL

We must test if user timespec is non-NULL before copying from userpace,
same as sys_recvmmsg().

Commiter note: changed it so that we have just one branch.

Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/compat.c

diff --git a/net/compat.c b/net/compat.c
index 6a2f75f..e1a56ad 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -758,9 +758,13 @@
 {
 	int datagrams;
 	struct timespec ktspec;
-	struct compat_timespec __user *utspec =
-			(struct compat_timespec __user *)timeout;
+	struct compat_timespec __user *utspec;
 
+	if (timeout == NULL)
+		return __sys_recvmmsg(fd, (struct mmsghdr __user *)mmsg, vlen,
+				      flags | MSG_CMSG_COMPAT, NULL);
+
+	utspec = (struct compat_timespec __user *)timeout;
 	if (get_user(ktspec.tv_sec, &utspec->tv_sec) ||
 	    get_user(ktspec.tv_nsec, &utspec->tv_nsec))
 		return -EFAULT;