CVE-2026-90136 in Linux
Summary
by MITRE • 09/17/2026
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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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified in the Linux kernel's AMD HSMP hardware monitoring subsystem represents a critical type conversion error that allows for the injection of malicious or nonsensical parameters into system management unit communications. Specifically, within the platform/x86/amd/hsmp driver, the function hsmp_hwmon_write() processes user-supplied values intended to set power caps for CPU sockets. The core technical flaw lies in how signed integer inputs are handled before being passed to hardware-specific messaging structures. When a user writes a negative value, such as -1000000, to the power1_cap sysfs interface, the code performs arithmetic division by MICROWATT_PER_MILLIWATT while treating the input as a signed long. This result is then assigned to msg.args[0], which is defined as an unsigned 32-bit integer (__u32). Because C language rules dictate that when performing operations between signed and unsigned types, or assigning negative values to unsigned variables, the bit pattern of the negative number is preserved but interpreted as a large positive value due to two's complement representation. Consequently, a small negative input like -1000000 can be converted into an extremely large unsigned integer, potentially exceeding several gigawatts when scaled by the conversion factor.
This type confusion leads directly to severe operational impacts on system stability and hardware integrity. The resulting massive power limit value is transmitted via the HSMP_SET_SOCKET_POWER_LIMIT command to the System Management Unit (SMU). Since this value far exceeds any realistic physical capability of the processor or motherboard, it effectively disables proper power capping mechanisms. In a worst-case scenario, if the SMU interprets such an out-of-bounds value as valid configuration data rather than rejecting it, it could lead to unpredictable hardware behavior, potential thermal runaway conditions due to lack of effective limits, or simply cause subsequent operations that rely on accurate power state tracking to fail. The vulnerability essentially bypasses input validation logic, allowing a local user with write access to the hwmon interface to send arbitrary and dangerous commands to low-level firmware components. This aligns with CWE-190, Integer Overflow or Wraparound, where arithmetic results exceed the range of the destination type, leading to unexpected behavior, as well as CWE-20, Improper Input Validation, because negative values are not explicitly rejected before processing.
From a threat modeling perspective using the MITRE ATT&CK framework, this vulnerability facilitates local privilege escalation scenarios or denial-of-service attacks through resource exhaustion and hardware misconfiguration. An attacker could exploit this to disrupt system stability by forcing the SMU into an invalid state, potentially causing kernel panics or requiring hard resets if the firmware fails to handle the erroneous command gracefully. Furthermore, it undermines the integrity of power management features that are critical for energy efficiency and thermal safety in modern computing environments. The lack of boundary checks on signed inputs before conversion to unsigned types is a common pitfall in systems programming, particularly when interfacing with hardware registers or communication protocols that expect strictly positive integers for configuration parameters like power limits.
The resolution involves implementing explicit validation logic within the hsmp_hwmon_write() function to reject negative values prior to any arithmetic operations or type conversions. By checking if the input value is less than zero and returning -EINVAL, the kernel ensures that only valid, non-negative power cap settings are processed and transmitted to the SMU. This fix restores the integrity of the hardware monitoring interface by enforcing strict data typing rules at the point of entry from user space. It prevents the conversion of negative numbers into large unsigned integers, thereby eliminating the possibility of sending multi-gigawatt limits that could destabilize the system. The patch maintains backward compatibility for valid positive writes while closing the security gap that allowed invalid inputs to propagate through the driver stack. This approach exemplifies secure coding practices by validating input constraints early in the processing pipeline, ensuring that downstream components receive only well-formed and logically consistent data.