fuse: do not use iocb after it may have been freed

commit 7cabc61e upstream.

There's a race in fuse_direct_IO(), whereby is_sync_kiocb() is called on an
iocb that could have been freed if async io has already completed.  The fix
in this case is simple and obvious: cache the result before starting io.

It was discovered by KASan:

kernel: ==================================================================
kernel: BUG: KASan: use after free in fuse_direct_IO+0xb1a/0xcc0 at addr ffff88036c414390
Signed-off-by: Robert Doebbelin <robert@quobyte.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: bcba24cc ("fuse: enable asynchronous processing direct IO")
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/fuse/file.c

@@ -2876,6 +2876,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,
 	loff_t i_size;
 	size_t count = iov_iter_count(iter);
 	struct fuse_io_priv *io;
+	bool is_sync = is_sync_kiocb(iocb);
 
 	pos = offset;
 	inode = file->f_mapping->host;
@@ -2915,7 +2916,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,
 	 * to wait on real async I/O requests, so we must submit this request
 	 * synchronously.
 	 */
-	if (!is_sync_kiocb(iocb) && (offset + count > i_size) && rw == WRITE)
+	if (!is_sync && (offset + count > i_size) && rw == WRITE)
 		io->async = false;
 
 	if (rw == WRITE)
@@ -2927,7 +2928,7 @@ fuse_direct_IO(int rw, struct kiocb *iocb, struct iov_iter *iter,
 		fuse_aio_complete(io, ret < 0 ? ret : 0, -1);
 
 		/* we have a non-extending, async request, so return */
-		if (!is_sync_kiocb(iocb))
+		if (!is_sync)
 			return -EIOCBQUEUED;
 
 		ret = wait_on_sync_kiocb(iocb);