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Commit a056bdaa authored by Jan Kara's avatar Jan Kara Committed by Theodore Ts'o
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ext4: fix data corruption for mmap writes 



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: default avatarMichael Zimmer <michael@swarm64.com>
CC: stable@vger.kernel.org
Fixes: cb20d518
Signed-off-by: default avatarJan Kara <jack@suse.cz>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 4f8caa60

fs/ext4/inode.c

+16 −2
Original line number Diff line number Diff line
@@ -2124,15 +2124,29 @@ static int ext4_writepage(struct page *page, 
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);

	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;

	clear_page_dirty_for_io(page);

	err = ext4_bio_write_page(&mpd->io_submit, page, len, mpd->wbc, false);

	if (!err)

		mpd->wbc->nr_to_write--;