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    Subject[PATCH 5.9 058/255] xfs: flush new eof page on truncate to avoid post-eof corruption
    Date
    From: Brian Foster <bfoster@redhat.com>

    [ Upstream commit 869ae85dae64b5540e4362d7fe4cd520e10ec05c ]

    It is possible to expose non-zeroed post-EOF data in XFS if the new
    EOF page is dirty, backed by an unwritten block and the truncate
    happens to race with writeback. iomap_truncate_page() will not zero
    the post-EOF portion of the page if the underlying block is
    unwritten. The subsequent call to truncate_setsize() will, but
    doesn't dirty the page. Therefore, if writeback happens to complete
    after iomap_truncate_page() (so it still sees the unwritten block)
    but before truncate_setsize(), the cached page becomes inconsistent
    with the on-disk block. A mapped read after the associated page is
    reclaimed or invalidated exposes non-zero post-EOF data.

    For example, consider the following sequence when run on a kernel
    modified to explicitly flush the new EOF page within the race
    window:

    $ xfs_io -fc "falloc 0 4k" -c fsync /mnt/file
    $ xfs_io -c "pwrite 0 4k" -c "truncate 1k" /mnt/file
    ...
    $ xfs_io -c "mmap 0 4k" -c "mread -v 1k 8" /mnt/file
    00000400: 00 00 00 00 00 00 00 00 ........
    $ umount /mnt/; mount <dev> /mnt/
    $ xfs_io -c "mmap 0 4k" -c "mread -v 1k 8" /mnt/file
    00000400: cd cd cd cd cd cd cd cd ........

    Update xfs_setattr_size() to explicitly flush the new EOF page prior
    to the page truncate to ensure iomap has the latest state of the
    underlying block.

    Fixes: 68a9f5e7007c ("xfs: implement iomap based buffered write path")
    Signed-off-by: Brian Foster <bfoster@redhat.com>
    Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    fs/xfs/xfs_iops.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
    index 80a13c8561d85..bf93a7152181c 100644
    --- a/fs/xfs/xfs_iops.c
    +++ b/fs/xfs/xfs_iops.c
    @@ -911,6 +911,16 @@ xfs_setattr_size(
    error = iomap_zero_range(inode, oldsize, newsize - oldsize,
    &did_zeroing, &xfs_buffered_write_iomap_ops);
    } else {
    + /*
    + * iomap won't detect a dirty page over an unwritten block (or a
    + * cow block over a hole) and subsequently skips zeroing the
    + * newly post-EOF portion of the page. Flush the new EOF to
    + * convert the block before the pagecache truncate.
    + */
    + error = filemap_write_and_wait_range(inode->i_mapping, newsize,
    + newsize);
    + if (error)
    + return error;
    error = iomap_truncate_page(inode, newsize, &did_zeroing,
    &xfs_buffered_write_iomap_ops);
    }
    --
    2.27.0


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