CVE-2025-38066 dm cache: prevent BUG_ON by blocking retries on failed device resumes
A local attacker can trigger a kernel BUG_ON crash by repeatedly resuming a corrupted dm-cache device, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-38066?
CVE-2025-38066 is a medium-severity denial of service vulnerability in Debian Debian Linux, published on 18 Jun 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
dm cache: prevent BUG_ON by blocking retries on failed device resumes
A cache device failing to resume due to mapping errors should not be retried, as the failure leaves a partially initialized policy object. Repeating the resume operation risks triggering BUG_ON when reloading cache mappings into the incomplete policy object.
Reproduce steps:
1. create a cache metadata consisting of 512 or more cache blocks, with some mappings stored in the first array block of the mapping array. Here we use cache_restore v1.0 to build the metadata.
cat <<EOF >> cmeta.xml <superblock uuid="" block_size="128" nr_cache_blocks="512" \ policy="smq" hint_width="4"> <mappings> <mapping cache_block="0" origin_block="0" dirty="false"/> </mappings> </superblock> EOF dmsetup create cmeta --table "0 8192 linear /dev/sdc 0" cache_restore -i cmeta.xml -o /dev/mapper/cmeta --metadata-version=2 dmsetup remove cmeta
2. wipe the second array block of the mapping array to simulate data degradations.
mapping_root=$(dd if=/dev/sdc bs=1c count=8 skip=192 \ 2>/dev/null | hexdump -e '1/8 "%u\n"') ablock=$(dd if=/dev/sdc bs=1c count=8 skip=$((4096*mapping_root+2056)) \ 2>/dev/null | hexdump -e '1/8 "%u\n"') dd if=/dev/zero of=/dev/sdc bs=4k count=1 seek=$ablock
3. try bringing up the cache device. The resume is expected to fail due to the broken array block.
dmsetup create cmeta --table "0 8192 linear /dev/sdc 0" dmsetup create cdata --table "0 65536 linear /dev/sdc 8192" dmsetup create corig --table "0 524288 linear /dev/sdc 262144" dmsetup create cache --notable dmsetup load cache --table "0 524288 cache /dev/mapper/cmeta \ /dev/mapper/cdata /dev/mapper/corig 128 2 metadata2 writethrough smq 0" dmsetup resume cache
4. try resuming the cache again. An unexpected BUG_ON is triggered while loading cache mappings.
dmsetup resume cache
Kernel logs:
(snip) ------------[ cut here ]------------ kernel BUG at drivers/md/dm-cache-policy-smq.c:752! Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI CPU: 0 UID: 0 PID: 332 Comm: dmsetup Not tainted 6.13.4 #3 RIP: 0010:smq_load_mapping+0x3e5/0x570
Fix by disallowing resume operations for devices that failed the initial attempt.
Weakness: CWE-617
Which versions are affected by CVE-2025-38066?
It affects 2 products.
Products include Debian Debian Linux, Linux kernel.
How do I fix CVE-2025-38066?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2025-38066 being exploited?
There is no known exploitation of CVE-2025-38066 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as AZL-63938, BELL-CVE-2025-38066, DEBIAN-CVE-2025-38066, GHSA-97x5-wjv7-8w72, ROOT-OS-DEBIAN-11-CVE-2025-38066, ROOT-OS-DEBIAN-12-CVE-2025-38066 and 3 more.