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    Subject[PATCH 5.0 34/52] ext4: fix data corruption caused by unaligned direct AIO
    Date
    5.0-stable review patch.  If anyone has any objections, please let me know.

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

    From: Lukas Czerner <lczerner@redhat.com>

    commit 372a03e01853f860560eade508794dd274e9b390 upstream.

    Ext4 needs to serialize unaligned direct AIO because the zeroing of
    partial blocks of two competing unaligned AIOs can result in data
    corruption.

    However it decides not to serialize if the potentially unaligned aio is
    past i_size with the rationale that no pending writes are possible past
    i_size. Unfortunately if the i_size is not block aligned and the second
    unaligned write lands past i_size, but still into the same block, it has
    the potential of corrupting the previous unaligned write to the same
    block.

    This is (very simplified) reproducer from Frank

    // 41472 = (10 * 4096) + 512
    // 37376 = 41472 - 4096

    ftruncate(fd, 41472);
    io_prep_pwrite(iocbs[0], fd, buf[0], 4096, 37376);
    io_prep_pwrite(iocbs[1], fd, buf[1], 4096, 41472);

    io_submit(io_ctx, 1, &iocbs[1]);
    io_submit(io_ctx, 1, &iocbs[2]);

    io_getevents(io_ctx, 2, 2, events, NULL);

    Without this patch the 512B range from 40960 up to the start of the
    second unaligned write (41472) is going to be zeroed overwriting the data
    written by the first write. This is a data corruption.

    00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    *
    00009200 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
    *
    0000a000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    *
    0000a200 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31

    With this patch the data corruption is avoided because we will recognize
    the unaligned_aio and wait for the unwritten extent conversion.

    00000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
    *
    00009200 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
    *
    0000a200 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
    *
    0000b200

    Reported-by: Frank Sorenson <fsorenso@redhat.com>
    Signed-off-by: Lukas Czerner <lczerner@redhat.com>
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Fixes: e9e3bcecf44c ("ext4: serialize unaligned asynchronous DIO")
    Cc: stable@vger.kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/ext4/file.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/fs/ext4/file.c
    +++ b/fs/ext4/file.c
    @@ -125,7 +125,7 @@ ext4_unaligned_aio(struct inode *inode,
    struct super_block *sb = inode->i_sb;
    int blockmask = sb->s_blocksize - 1;

    - if (pos >= i_size_read(inode))
    + if (pos >= ALIGN(i_size_read(inode), sb->s_blocksize))
    return 0;

    if ((pos | iov_iter_alignment(from)) & blockmask)

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