From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.linuxfoundation.org ([140.211.169.12]:44970 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932711AbdJJTye (ORCPT ); Tue, 10 Oct 2017 15:54:34 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, Michael Zimmer , Jan Kara , Theodore Tso Subject: [PATCH 4.4 45/47] ext4: fix data corruption for mmap writes Date: Tue, 10 Oct 2017 21:50:58 +0200 Message-Id: <20171010195021.404916605@linuxfoundation.org> In-Reply-To: <20171010195019.584170396@linuxfoundation.org> References: <20171010195019.584170396@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: stable-owner@vger.kernel.org List-ID: 4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Jan Kara commit a056bdaae7a181f7dcc876cfab2f94538e508709 upstream. mpage_submit_page() can race with another process growing i_size and writing data via mmap to the written-back page. As mpage_submit_page() samples i_size too early, it may happen that ext4_bio_write_page() zeroes out too large tail of the page and thus corrupts user data. Fix the problem by sampling i_size only after the page has been write-protected in page tables by clear_page_dirty_for_io() call. Reported-by: Michael Zimmer CC: stable@vger.kernel.org Fixes: cb20d5188366f04d96d2e07b1240cc92170ade40 Signed-off-by: Jan Kara Signed-off-by: Theodore Ts'o Signed-off-by: Greg Kroah-Hartman --- fs/ext4/inode.c | 24 +++++++++++++++++++----- 1 file changed, 19 insertions(+), 5 deletions(-) --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -1946,15 +1946,29 @@ static int ext4_writepage(struct page *p static int mpage_submit_page(struct mpage_da_data *mpd, struct page *page) { int len; - loff_t size = i_size_read(mpd->inode); + loff_t size; int err; BUG_ON(page->index != mpd->first_page); - if (page->index == size >> PAGE_CACHE_SHIFT) - len = size & ~PAGE_CACHE_MASK; - else - len = PAGE_CACHE_SIZE; clear_page_dirty_for_io(page); + /* + * We have to be very careful here! Nothing protects writeback path + * against i_size changes and the page can be writeably mapped into + * page tables. So an application can be growing i_size and writing + * data through mmap while writeback runs. clear_page_dirty_for_io() + * write-protects our page in page tables and the page cannot get + * written to again until we release page lock. So only after + * clear_page_dirty_for_io() we are safe to sample i_size for + * ext4_bio_write_page() to zero-out tail of the written page. We rely + * on the barrier provided by TestClearPageDirty in + * clear_page_dirty_for_io() to make sure i_size is really sampled only + * after page tables are updated. + */ + size = i_size_read(mpd->inode); + if (page->index == size >> PAGE_SHIFT) + len = size & ~PAGE_MASK; + else + len = PAGE_SIZE; err = ext4_bio_write_page(&mpd->io_submit, page, len, mpd->wbc, false); if (!err) mpd->wbc->nr_to_write--;