io_uring/kbuf: use WRITE_ONCE() for userspace-shared buffer ring fields

buf->addr and buf->len reside in memory shared with userspace. They
should be written with WRITE_ONCE() to guarantee atomic stores and
prevent tearing or other unsafe compiler optimizations.

Signed-off-by: Joanne Koong <[e-mail]>
Cc: Caleb Sander Mateos <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

io_uring/kbuf.c

···
 		buf_len -= this_len;
 		/* Stop looping for invalid buffer length of 0 */
 		if (buf_len || !this_len) {
-			buf->addr = READ_ONCE(buf->addr) + this_len;
-			buf->len = buf_len;
+			WRITE_ONCE(buf->addr, READ_ONCE(buf->addr) + this_len);
+			WRITE_ONCE(buf->len, buf_len);
 			return false;
 		}
-		buf->len = 0;
+		WRITE_ONCE(buf->len, 0);
 		bl->head++;
 		len -= this_len;
 	}
···
 				arg->partial_map = 1;
 				if (iov != arg->iovs)
 					break;
-				buf->len = len;
+				WRITE_ONCE(buf->len, len);
 			}
 		}