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