uml: fix request->sector update

It is theoretically possible for a request to finish and be freed between
writing it to the I/O thread and updating the sector count.  In this case, the
update will dereference a freed pointer.

To avoid this, I delay the update until processing the next sg segment, when
the request pointer is known to be good.
Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/ubd_kern.c

@@ -1083,7 +1083,7 @@ static void do_ubd_request(request_queue_t *q)
 {
 	struct io_thread_req *io_req;
 	struct request *req;
-	int n;
+	int n, last_sectors;
 
 	while(1){
 		struct ubd *dev = q->queuedata;
@@ -1099,9 +1099,11 @@ static void do_ubd_request(request_queue_t *q)
 		}
 
 		req = dev->request;
+		last_sectors = 0;
 		while(dev->start_sg < dev->end_sg){
 			struct scatterlist *sg = &dev->sg[dev->start_sg];
 
+			req->sector += last_sectors;
 			io_req = kmalloc(sizeof(struct io_thread_req),
 					 GFP_ATOMIC);
 			if(io_req == NULL){
@@ -1113,6 +1115,7 @@ static void do_ubd_request(request_queue_t *q)
 					(unsigned long long) req->sector << 9,
 					sg->offset, sg->length, sg->page);
 
+			last_sectors = sg->length >> 9;
 			n = os_write_file(thread_fd, &io_req,
 					  sizeof(struct io_thread_req *));
 			if(n != sizeof(struct io_thread_req *)){
@@ -1124,7 +1127,6 @@ static void do_ubd_request(request_queue_t *q)
 				return;
 			}
 
-			req->sector += sg->length >> 9;
 			dev->start_sg++;
 		}
 		dev->end_sg = 0;