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Subject[PATCH 3.2 051/106] ext4: fix data corruption for mmap writes
3.2.93-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@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 <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>
---
fs/ext4/inode.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1344,7 +1344,6 @@ static int mpage_da_submit_io(struct mpa
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 mpa
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 mpa
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
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