From: Jan Kara Date: Fri, 26 May 2017 21:45:45 +0000 (-0400) Subject: ext4: fix data corruption for mmap writes X-Git-Tag: v4.4.92~3 X-Git-Url: http://git.hungrycats.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=40c00e5fac3abbd5e6fa08e93fa99b3e632ece16;p=linux ext4: fix data corruption for mmap writes 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 --- diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 1796d1bd9a1d..194a6baa4283 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -1946,15 +1946,29 @@ 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); - 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--;