drivers/rapidio/rio_cm.c: prevent possible heap overwrite

In

riocm_cdev_ioctl(RIO_CM_CHAN_SEND)
-> cm_chan_msg_send()
-> riocm_ch_send()

cm_chan_msg_send() checks that userspace didn't send too much data but
riocm_ch_send() failed to check that userspace sent sufficient data. The
result is that riocm_ch_send() can write to fields in the rio_ch_chan_hdr
which were outside the bounds of the space which cm_chan_msg_send()
allocated.

Address this by teaching riocm_ch_send() to check that the entire
rio_ch_chan_hdr was copied in from userspace.

Reported-by: maher azz <[e-mail]>
Cc: Matt Porter <[e-mail]>
Cc: Alexandre Bounine <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

drivers/rapidio/rio_cm.c

···
 	if (buf == NULL || ch_id == 0 || len == 0 || len > RIO_MAX_MSG_SIZE)
 		return -EINVAL;
 
+	if (len < sizeof(struct rio_ch_chan_hdr))
+		return -EINVAL;		/* insufficient data from user */
+
 	ch = riocm_get_channel(ch_id);
 	if (!ch) {
 		riocm_error("%s(%d) ch_%d not found", current->comm,