NFS: Fix a use after free in nfs_direct_join_group()

Be more careful when tearing down the subrequests of an O_DIRECT write
as part of a retransmission.
Reported-by: Chris Mason <clm@fb.com>
Fixes: ed5d588f ("NFS: Try to join page groups before an O_DIRECT retransmission")
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/direct.c

@@ -472,20 +472,26 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, struct iov_iter *iter,
 	return result;
 }
 
-static void
-nfs_direct_join_group(struct list_head *list, struct inode *inode)
+static void nfs_direct_join_group(struct list_head *list, struct inode *inode)
 {
-	struct nfs_page *req, *next;
+	struct nfs_page *req, *subreq;
 
 	list_for_each_entry(req, list, wb_list) {
-		if (req->wb_head != req || req->wb_this_page == req)
+		if (req->wb_head != req)
 			continue;
-		for (next = req->wb_this_page;
-				next != req->wb_head;
-				next = next->wb_this_page) {
-			nfs_list_remove_request(next);
-			nfs_release_request(next);
-		}
+		subreq = req->wb_this_page;
+		if (subreq == req)
+			continue;
+		do {
+			/*
+			 * Remove subrequests from this list before freeing
+			 * them in the call to nfs_join_page_group().
+			 */
+			if (!list_empty(&subreq->wb_list)) {
+				nfs_list_remove_request(subreq);
+				nfs_release_request(subreq);
+			}
+		} while ((subreq = subreq->wb_this_page) != req);
 		nfs_join_page_group(req, inode);
 	}
 }