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 <michael@swarm64.com>
Fixes: cb20d5188366f04d96d2e07b1240cc92170ade40
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2: The writeback path is very different here and
 it needs to read i_size long before calling clear_page_dirty_for_io().
 So read it twice and skip the page if it changed.]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index bee0d76..fde77a7 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1344,7 +1344,6 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
        int ret = 0, err, nr_pages, i;
        struct inode *inode = mpd->inode;
        struct address_space *mapping = inode->i_mapping;
-       loff_t size = i_size_read(inode);
        unsigned int len, block_start;
        struct buffer_head *bh, *page_bufs = NULL;
        int journal_data = ext4_should_journal_data(inode);
@@ -1370,6 +1369,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
                for (i = 0; i < nr_pages; i++) {
                        int commit_write = 0, skip_page = 0;
                        struct page *page = pvec.pages[i];
+                       loff_t size = i_size_read(inode);
 
                        index = page->index;
                        if (index > end)
@@ -1443,11 +1443,31 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
                        if (skip_page)
                                goto skip_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.
+                        */
+                       if (size != i_size_read(inode)) {
+                               set_page_dirty(page);
+                               goto skip_page;
+                       }
+
                        if (commit_write)
                                /* mark the buffer_heads as dirty & uptodate */
                                block_commit_write(page, 0, len);
-
-                       clear_page_dirty_for_io(page);
                        /*
                         * Delalloc doesn't support data journalling,
                         * but eventually maybe we'll lift this