The corruption occurs when a BAT entry aligned to 4096 bytes is changed. Specifically, the corruption occurs during the creation of the LOG Data Descriptor. The incorrect behavior involves copying 4088 bytes from the original 4096 bytes aligned offset to `tmp[8..4096]` and then copying the new value for the first BAT entry to the beginning `tmp[0..8]`. This results in all existing BAT entries inside the 4K region being incorrectly moved by 8 bytes and the last entry being lost.
This bug did not cause noticeable corruption when only sequentially writing once to an empty dynamic VHDX (e.g. using `qemu-img convert -O vhdx -o subformat=dynamic ...`), but it still resulted in invalid values for the (unused) Sector Bitmap BAT entries. Importantly, this corruption would only become noticeable after the corrupted BAT is re-read from the file. Resolves: https://gitlab.com/qemu-project/qemu/-/issues/727 Signed-off-by: Lukas Tschoke <lukts...@gmail.com> --- block/vhdx-log.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/vhdx-log.c b/block/vhdx-log.c index c48cf65d62..38148f107a 100644 --- a/block/vhdx-log.c +++ b/block/vhdx-log.c @@ -981,7 +981,7 @@ static int vhdx_log_write(BlockDriverState *bs, BDRVVHDXState *s, sector_write = merged_sector; } else if (i == sectors - 1 && trailing_length) { /* partial sector at the end of the buffer */ - ret = bdrv_pread(bs->file, file_offset, + ret = bdrv_pread(bs->file, file_offset + trailing_length, VHDX_LOG_SECTOR_SIZE - trailing_length, merged_sector + trailing_length, 0); if (ret < 0) { -- 2.40.0
