scsi: scsi_transport_fc: Adjust struct fc_nl_event flex array usage

In order to help the compiler reason about the destination buffer in struct
fc_nl_event, add a flexible array member for this purpose. However, since
the header is UAPI, it must not change size or layout, so a union is used.

The allocation size calculations are also corrected (it was potentially
allocating an extra 8 bytes), and the padding is zeroed to avoid leaking
kernel heap memory contents.

Detected at run-time by the recently added memcpy() bounds checking:

memcpy: detected field-spanning write (size 8) of single field "&event->event_data" at drivers/scsi/scsi_transport_fc.c:581 (size 4)

Link: https://lore.kernel.org/linux-next/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Cc: "James E.J. Bottomley" <[e-mail]>
Cc: "Martin K. Petersen" <[e-mail]>
Cc: [e-mail]
Reported-by: Sachin Sant <[e-mail]>
Tested-by: Sachin Sant <[e-mail]>
Reviewed-by: James Smart <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/scsi_transport_fc.c

···
 	struct nlmsghdr	*nlh;
 	struct fc_nl_event *event;
 	const char *name;
-	u32 len;
+	size_t len, padding;
 	int err;
 
 	if (!data_buf || data_len < 4)
···
 		goto send_fail;
 	}
 
-	len = FC_NL_MSGALIGN(sizeof(*event) + data_len);
+	len = FC_NL_MSGALIGN(sizeof(*event) - sizeof(event->event_data) + data_len);
 
 	skb = nlmsg_new(len, GFP_KERNEL);
 	if (!skb) {
···
 	event->event_num = event_number;
 	event->event_code = event_code;
 	if (data_len)
-		memcpy(&event->event_data, data_buf, data_len);
+		memcpy(event->event_data_flex, data_buf, data_len);
+	padding = len - offsetof(typeof(*event), event_data_flex) - data_len;
+	memset(event->event_data_flex + data_len, 0, padding);
 
 	nlmsg_multicast(scsi_nl_sock, skb, 0, SCSI_NL_GRP_FC_EVENTS,
 			GFP_KERNEL);

include/uapi/scsi/scsi_netlink_fc.h

···
  * FC Transport Broadcast Event Message :
  *   FC_NL_ASYNC_EVENT
  *
- * Note: if Vendor Unique message, &event_data will be  start of
+ * Note: if Vendor Unique message, event_data_flex will be start of
  * 	 vendor unique payload, and the length of the payload is
  *       per event_datalen
  *
···
 	__u16 event_datalen;
 	__u32 event_num;
 	__u32 event_code;
-	__u32 event_data;
+	union {
+		__u32 event_data;
+		__DECLARE_FLEX_ARRAY(__u8, event_data_flex);
+	};
 } __attribute__((aligned(sizeof(__u64))));