CVE-2025-21840 thermal/netlink: Prevent userspace segmentation fault by adjusting UAPI header
A kernel thermal netlink header change caused userspace segmentation faults in tools like intel-lpmd, leading to 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-2025-21840?
CVE-2025-21840 is a medium-severity vulnerability in Linux kernel, published on 7 Mar 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:
thermal/netlink: Prevent userspace segmentation fault by adjusting UAPI header
The intel-lpmd tool [1], which uses the THERMAL_GENL_ATTR_CPU_CAPABILITY attribute to receive HFI events from kernel space, encounters a segmentation fault after commit 1773572863c4 ("thermal: netlink: Add the commands and the events for the thresholds").
The issue arises because the THERMAL_GENL_ATTR_CPU_CAPABILITY raw value was changed while intel_lpmd still uses the old value.
Although intel_lpmd can be updated to check the THERMAL_GENL_VERSION and use the appropriate THERMAL_GENL_ATTR_CPU_CAPABILITY value, the commit itself is questionable.
The commit introduced a new element in the middle of enum thermal_genl_attr, which affects many existing attributes and introduces potential risks and unnecessary maintenance burdens for userspace thermal netlink event users.
Solve the issue by moving the newly introduced THERMAL_GENL_ATTR_TZ_PREV_TEMP attribute to the end of the enum thermal_genl_attr. This ensures that all existing thermal generic netlink attributes remain unaffected.
[ rjw: Subject edits ]
Which versions are affected by CVE-2025-21840?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21840?
Apply the vendor's patch.
Is CVE-2025-21840 being exploited?
There is no known exploitation of CVE-2025-21840 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-21840, GHSA-pcf3-xr6w-mj6g, UBUNTU-CVE-2025-21840.