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

[Qianheng Peng]

Hello Zhongqiu,
Thanks for review.

On 7/14/2026 19:08:34, Zhongqiu Han wrote:
>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@xxxxxxxxxxxxxxx>
>>>> Suggested-by: Xibo Wang <wangxb12@xxxxxxxxxxxxxxx>
>>>> Signed-off-by: Qianheng Peng <pengqh1@xxxxxxxxxxxxxxx>
>>>> ---
>>>>   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().
This detection really make sense, I will post it in patch v2.

-- 
Thanks
Qianheng