4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jan Kara <j...@suse.cz>
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 <mich...@swarm64.com>
CC: sta...@vger.kernel.org
Fixes: cb20d5188366f04d96d2e07b1240cc92170ade40
Signed-off-by: Jan Kara <j...@suse.cz>
Signed-off-by: Theodore Ts'o <ty...@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org>
---
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--;
