Re: [BUG] general protection fault in cpuset_cpus_allowed

[Ridong Chen <ridong.chen@linux.dev>]

On 8/28/2026 5:21 PM, Ridong Chen wrote:
> 
> 
> On 8/28/2026 1:43 PM, Farhad Alemi wrote:
>> Hello,
>>
>> As part of the kernel research at ASU's SEFCOM
>> lab, we hit the crash below. Crash reports can be found here:
>>
>>    https://github.com/farhad-alemi/public_bug_reports/tree/main/166-general- 
>> protection-fault-in-cpuset-cpus-allowed/
>>
>>    Oops: general protection fault, probably for non-canonical address
>> 0xdffffc000000001d: 0000 [#1] SMP KASAN NOPTI
>>    KASAN: null-ptr-deref in range [0x00000000000000e8-0x00000000000000ef]
>>    RIP: 0010:bitmap_intersects include/linux/bitmap.h:440 [inline]
>>    RIP: 0010:cpumask_intersects include/linux/cpumask.h:822 [inline]
>>    RIP: 0010:guarantee_active_cpus kernel/cgroup/cpuset.c:481 [inline]
>>    RIP: 0010:__cpuset_cpus_allowed_locked kernel/cgroup/cpuset.c:4022 [inline]
>>    RIP: 0010:cpuset_cpus_allowed+0x14a/0x2f0 kernel/cgroup/cpuset.c:4071
>>    Call Trace:
>>     cpuset_cpus_allowed+0x14a/0x2f0 kernel/cgroup/cpuset.c:4071
>>     __sched_setaffinity+0x98/0x3c0 kernel/sched/syscalls.c:1149
>>     sched_setaffinity+0x341/0x4a0 kernel/sched/syscalls.c:1237
>>     __x64_sys_sched_setaffinity+0x106/0x170 kernel/sched/syscalls.c:1262
>>
>> A second capture reaches the identical fault -- same address, same KASAN range,
>> same RIP -- through cpuset_fork <- cgroup_post_fork <- copy_process <-
>> kernel_clone <- clone3, with the crashing task being systemd at PID 1.
>>
>> Our reproducer.c is available upon request.
>>
> 
> Thank you for the report.
> 
> Could you please share the reproducer.c with us?
> 

I suspect this might be caused by cs becoming NULL when cs = parent_cs(top_cpuset).

Having the reproducer would help me debug how this situation could occur.

```
static void guarantee_active_cpus(struct task_struct *tsk,
				  struct cpumask *pmask)
{
...
             while (!cpumask_intersects(cs->effective_cpus, pmask))
                     cs = parent_cs(cs);
...
}
```

>> Happy to test a patch if that would help.
>>
>> Regards,
> 

-- 
Best regards
Ridong