Re: [PATCH] cpufreq: amd-pstate-ut: fix null pointer dereference

[Zhongqiu Han <zhongqiu.han@oss.qualcomm.com>]

Hi Qianheng,

On 7/14/2026 6:22 PM, Qianheng Peng wrote:
> Hello K Prateek,
> Thanks for questions.
> 
> On 7/14/2026 14:39, K Prateek Nayak wrote:
>> On 7/14/2026 1:29 PM, Qianheng Peng wrote:
>>> The crash issue may occur when insmod amd_pstate_ut modules.
>>>
>>>     amd_pstate_ut: 1    amd_pstate_ut_acpi_cpc_valid  success!
>>>     amd_pstate_ut: 2    amd_pstate_ut_check_enabled   success!
>>>     BUG: kernel NULL pointer dereference, address: 0000000000000080
>>>     #PF: supervisor read access in kernel mode
>>>     #PF: error_code(0x0000) - not-present page
>>>     PGD 0 P4D 0
>>>     Oops: 0000 [#1] SMP NOPTI
>>>     CPU: 0 PID: 20300 Comm: modprobe
>>>     Kdump: loaded Tainted: G O 6.6.0-0010.rc1.ctl4.x86_64 #1
>>>     Hardware name: FiberHome R2200 V5/Xeon Boards, BIOS 3.1a 02/24/2020
>>
>> Is this an AMD platform?
> No, it's Intel(R) Xeon(R) Gold 5128.
> 
>>>     RIP: 0010:amd_pstate_ut_check_perf+0x141/0x280 [amd_pstate_ut]
>>>     Call Trace:
>>>      <TASK>
>>>      amd_pstate_ut_init+0x1b/0xff0 [amd_pstate_ut]
>>>      ? __pfx_amd_pstate_ut_init+0x10/0x10 [amd_pstate_ut]
>>>      do_one_initcall+0x42/0x2e0
>>>      ? kmalloc_trace+0x26/0x90
>>>      do_init_module+0x60/0x240
>>>      __se_sys_init_module+0x185/0x1c0
>>>      do_syscall_64+0x62/0x190
>>>      entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>>      </TASK>
>>>
>>> Add invalidation check of cpudata in amd_pstate_ut_check_perf() and
>>> amd_pstate_ut_check_freq() to avoid unpredicated null pointer dereference.
>>>
>>> Fixes: 14eb1c96e3a3 ("cpufreq: amd-pstate: Add test module for amd-pstate driver")
>>> Suggested-by: Li Xiong <xiongl24@chinatelecom.cn>
>>> Suggested-by: Xibo Wang <wangxb12@chinatelecom.cn>
>>> Signed-off-by: Qianheng Peng <pengqh1@chinatelecom.cn>
>>> ---
>>>   drivers/cpufreq/amd-pstate-ut.c | 8 ++++++++
>>>   1 file changed, 8 insertions(+)
>>>
>>> diff --git a/drivers/cpufreq/amd-pstate-ut.c b/drivers/cpufreq/amd-pstate-ut.c
>>> index 735b29f..3b24260 100644
>>> --- a/drivers/cpufreq/amd-pstate-ut.c
>>> +++ b/drivers/cpufreq/amd-pstate-ut.c
>>> @@ -157,6 +157,10 @@ static int amd_pstate_ut_check_perf(u32 index)
>>>                  if (!policy)
>>>                          continue;
>>>                  cpudata = policy->driver_data;
>>> +               if (!cpudata) {
>>> +                       pr_err("%s driver_data of %s is empty!\n", __func__, policy->kobj.name);
>>> +                       return -EINVAL;
>>> +               }
>>>
>>>                  if (get_shared_mem()) {
>>>                          ret = cppc_get_perf_caps(cpu, &cppc_perf);
>>> @@ -229,6 +233,10 @@ static int amd_pstate_ut_check_freq(u32 index)
>>>                  if (!policy)
>>>                          continue;
>>>                  cpudata = policy->driver_data;
>>> +               if (!cpudata) {
>>> +                       pr_err("%s driver_data of %s is empty!\n", __func__, policy->kobj.name);
>>> +                       return -EINVAL;
>>> +               }
>>
>> The only time I think this can happen is when the amd-pstate-ut races with a
>> the amd_pstate.*_cpu_exit() (probably as a result of concurrent mode switch)
>> or if you are running this without running amd-pstate as the cpufreq driver
>> (which the ut doesn't check for beforehand)
>>
>> My setup fails at this exact same point when I have the driver unloaded.
>> What is the output of following before you run into this issue:
>>
>>     cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
>>     cat /sys/devices/system/cpu/amd_pstate/status
>>
>> Are you doing any concurrent hotplug / mode switch?
>>
>>>
>>>                  if (!((policy->cpuinfo.max_freq >= cpudata->nominal_freq) &&
>>>                          (cpudata->nominal_freq > cpudata->lowest_nonlinear_freq) &&
> In this machine, I got output as follow:
> ======================================================================================
> [root@ ~]# cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver
> intel_cpufreq
> [root@ ~]# cat /sys/devices/system/cpu/amd_pstate/status
> cat: /sys/devices/system/cpu/amd_pstate/status: No such file or directory (os error 2)
> ======================================================================================
> Otherwise,I didn't do any concurrent hotplug / mode switch, just modprobe amd_pstate_ut.
> 

It may be worth adding driver name detection to this ut driver, for
example via cpufreq_get_current_driver().


-- 
Thx and BRs,
Zhongqiu Han