From 8af037fe4cfeb88bbcded3122cec2c5be0b90907 Mon Sep 17 00:00:00 2001
From: Lukas Tschoke <lukts330@gmail.com>
Date: Sat, 8 Apr 2023 00:11:38 +0200
Subject: [PATCH] block/vhdx: fix dynamic VHDX BAT corruption
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
Cc: qemu-stable@nongnu.org
Signed-off-by: Lukas Tschoke <lukts330@gmail.com>
Message-Id: <6cfb6d6b-adc5-7772-c8a5-6bae9a0ad668@gmail.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.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 c48cf65d62e..38148f107a9 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) {
--
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