Overview

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

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.