Linux EUVD-2026-38929

In the Linux kernel, the following vulnerability has been resolved:

dm cache: fix dirty mapping checking in passthrough mode switching

As mentioned in commit 9b1cc9f251af ("dm cache: share cache-metadata object across inactive and active DM tables"), dm-cache assumed table reload occurs after suspension, while LVM's table preload breaks this assumption. The dirty mapping check for passthrough mode was designed around this assumption and is performed during table creation, causing the check to fail with preload while metadata updates are ongoing. This risks loading dirty mappings into passthrough mode, resulting in data loss.

Reproduce steps:

1. Create a writeback cache with zero migration_threshold to produce

dirty mappings

dmsetup create cmeta --table "0 8192 linear /dev/sdc 0" dmsetup create cdata --table "0 131072 linear /dev/sdc 8192" dmsetup create corig --table "0 262144 linear /dev/sdc 262144" dd if=/dev/zero of=/dev/mapper/cmeta bs=4k count=1 oflag=direct dmsetup create cache --table "0 262144 cache /dev/mapper/cmeta \ /dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writeback smq \ 2 migration_threshold 0"

1. Preload a table in passthrough mode

dmsetup reload cache --table "0 262144 cache /dev/mapper/cmeta \ /dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 passthrough smq 0"

1. Write to the first cache block to make it dirty

fio --filename=/dev/mapper/cache --name=populate --rw=write --bs=4k \ --direct=1 --size=64k

1. Resume the inactive table. Now it's possible to load the dirty block

into passthrough mode.

dmsetup resume cache

Fix by moving the checks to the preresume phase to support table preloading. Also remove the unused function dm_cache_metadata_all_clean.