[NET]: Fix function put_cmsg() which may cause usr application memory overflow

When used function put_cmsg() to copy kernel information to user 
application memory, if the memory length given by user application is 
not enough, by the bad length calculate of msg.msg_controllen, 
put_cmsg() function may cause the msg.msg_controllen to be a large 
value, such as 0xFFFFFFF0, so the following put_cmsg() can also write 
data to usr application memory even usr has no valid memory to store 
this. This may cause usr application memory overflow.

int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
{
    struct cmsghdr __user *cm
        = (__force struct cmsghdr __user *)msg->msg_control;
    struct cmsghdr cmhdr;
    int cmlen = CMSG_LEN(len);
    ~~~~~~~~~~~~~~~~~~~~~
    int err;

    if (MSG_CMSG_COMPAT & msg->msg_flags)
        return put_cmsg_compat(msg, level, type, len, data);

    if (cm==NULL || msg->msg_controllen < sizeof(*cm)) {
        msg->msg_flags |= MSG_CTRUNC;
        return 0; /* XXX: return error? check spec. */
    }
    if (msg->msg_controllen < cmlen) {
    ~~~~~~~~~~~~~~~~~~~~~~~~
        msg->msg_flags |= MSG_CTRUNC;
        cmlen = msg->msg_controllen;
    }
    cmhdr.cmsg_level = level;
    cmhdr.cmsg_type = type;
    cmhdr.cmsg_len = cmlen;

    err = -EFAULT;
    if (copy_to_user(cm, &cmhdr, sizeof cmhdr))
        goto out;
    if (copy_to_user(CMSG_DATA(cm), data, cmlen - sizeof(struct cmsghdr)))
        goto out;
    cmlen = CMSG_SPACE(len);
~~~~~~~~~~~~~~~~~~~~~~~~~~~
    If MSG_CTRUNC flags is set, msg->msg_controllen is less than 
CMSG_SPACE(len), "msg->msg_controllen -= cmlen" will cause unsinged int 
type msg->msg_controllen to be a large value.
~~~~~~~~~~~~~~~~~~~~~~~~~~~
    msg->msg_control += cmlen;
    msg->msg_controllen -= cmlen;
    ~~~~~~~~~~~~~~~~~~~~~
    err = 0;
out:
    return err;
}

The same promble exists in put_cmsg_compat(). This patch can fix this 
problem.
Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/compat.c

@@ -254,6 +254,8 @@ int put_cmsg_compat(struct msghdr *kmsg, int level, int type, int len, void *dat
 	if (copy_to_user(CMSG_COMPAT_DATA(cm), data, cmlen - sizeof(struct compat_cmsghdr)))
 		return -EFAULT;
 	cmlen = CMSG_COMPAT_SPACE(len);
+	if (kmsg->msg_controllen < cmlen)
+		cmlen = kmsg->msg_controllen;
 	kmsg->msg_control += cmlen;
 	kmsg->msg_controllen -= cmlen;
 	return 0;

net/core/scm.c

@@ -196,6 +196,8 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 	if (copy_to_user(CMSG_DATA(cm), data, cmlen - sizeof(struct cmsghdr)))
 		goto out;
 	cmlen = CMSG_SPACE(len);
+	if (msg->msg_controllen < cmlen)
+		cmlen = msg->msg_controllen;
 	msg->msg_control += cmlen;
 	msg->msg_controllen -= cmlen;
 	err = 0;