ceph: fix error handling in ceph_read_iter

BugLink: http://bugs.launchpad.net/bugs/1637510

commit 0d7718f6 upstream.

In case __ceph_do_getattr returns an error and the retry_op in
ceph_read_iter is not READ_INLINE, then it's possible to invoke
__free_page on a page which is NULL, this naturally leads to a crash.
This can happen when, for example, a process waiting on a MDS reply
receives sigterm.

Fix this by explicitly checking whether the page is set or not.
Signed-off-by: Nikolay Borisov <kernel@kyup.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

fs/ceph/file.c

@@ -929,7 +929,8 @@ static ssize_t ceph_read_iter(struct kiocb *iocb, struct iov_iter *to)
 		statret = __ceph_do_getattr(inode, page,
 					    CEPH_STAT_CAP_INLINE_DATA, !!page);
 		if (statret < 0) {
-			 __free_page(page);
+			if (page)
+				__free_page(page);
 			if (statret == -ENODATA) {
 				BUG_ON(retry_op != READ_INLINE);
 				goto again;