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