CVE-2026-90136 platform/x86/amd/hsmp: Reject negative power cap writes in hwmon
A local attacker with root privileges can write negative values to the AMD HSMP power cap, causing an integer overflow that sends invalid multi-gigawatt limits to the SMU.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-90136?
CVE-2026-90136 is a vulnerability in Linux, published on 17 Sep 2026. 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:
platform/x86/amd/hsmp: Reject negative power cap writes in hwmon
hsmp_hwmon_write() takes the user-supplied hwmon value as a signed long and assigns "val / MICROWATT_PER_MILLIWATT" to msg.args[0], which is a __u32. MICROWATT_PER_MILLIWATT is an unsigned long, so a negative write to power1_cap (e.g. "echo -1 > power1_cap") is first converted to a huge unsigned value by the division and then stored into the u32 argument.
As a result a nonsensical, multi-gigawatt socket power limit is sent to the SMU via HSMP_SET_SOCKET_POWER_LIMIT instead of the write being rejected.
Reject negative values with -EINVAL before the conversion.
Tested with HSMP enabled:
CAP=$(dirname $(grep -l amd_hsmp_hwmon \ /sys/class/hwmon/hwmon*/name | head -1))/power1_cap
# negative write echo -1000000 > $CAP ; echo "ret=$?" # valid positive write must still work echo 400000000 > $CAP ; echo "ret=$?"
Before: # echo -1000000 > $CAP ; echo "ret=$?" ret=0 <- accepted; bogus limit sent to SMU # echo 400000000 > $CAP ; echo "ret=$?" ret=0
After: # echo -1000000 > $CAP ; echo "ret=$?" bash: echo: write error: Invalid argument ret=1 <- rejected with -EINVAL # echo 400000000 > $CAP ; echo "ret=$?" ret=0 <- valid write still works
Which versions are affected by CVE-2026-90136?
How do I fix CVE-2026-90136?
Apply the vendor's patch.
Is CVE-2026-90136 being exploited?
There is no known exploitation of CVE-2026-90136 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-90136, DEBIAN-CVE-2026-90136, GHSA-547r-p462-qv3c, UBUNTU-CVE-2026-90136.