CVE-2026-90136

Published Sep 17, 2026

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

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References

Frequently Asked Questions

What is CVE-2026-90136? +
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
How do I check if I'm vulnerable to CVE-2026-90136? +
You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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