Re: [PATCH] intel_th: core: fix null pointer dereference in intel_th_irq

[David Arcari <darcari@redhat.com>]

Hi Alex,

On 9/26/25 12:19 PM, alex@ash.works wrote:
> On 2025-08-25 20:45, David Arcari wrote:
>> In certain cases intel_th_irq can reference a null entry in
>> the th->thdev array.  This results in the splat shown below.
>> The problem is that intel_th_output_enable() can modify the
>> thdev[] array at the same time intel_th_irq is referencing
>> the same array.  This can be fixed by disabling interrupts
>> during the call to intel_th_output_enable().
> 
> Hi David,
> 
> Thank you for the bug report and rootcausing! Can you please also
> detail the sequence of actions by which this is reproduced, so
> that I can test my fix and not bother you with a back-and-forth
> over-email debugging and also add it to our regression testing?
> Doesn't have to be a shell script (although I wouldn't say no
> to that), plain english would work in a pinch. If you have the
> time, I'm also curious about your use case for intel_th.

Unfortunately, I don't have a great reproducer.  I have a system which 
is afflicted in ~ 1 out of every 100 reboots.  Adding debug code made 
the problem easier to reproduce.

> 
> This has eluded our testing for about 10 years, so I'm very
> interested in the reproducer.
> 
>> BUG: kernel NULL pointer dereference, address: 0000000000000304
>> Oops: Oops: 0000 [#1] SMP NOPTI
>> RIP: 0010:intel_th_irq+0x26/0x70 [intel_th]
> 
> Yes, this is absolutely a bug.
> 
>> @@ -715,7 +715,9 @@ intel_th_subdevice_alloc(struct intel_th *th,
>>  int intel_th_output_enable(struct intel_th *th, unsigned int otype)
>>  {
>>      struct intel_th_device *thdev;
>> -    int src = 0, dst = 0;
>> +    int src = 0, dst = 0, ret = 0;
>> +
>> +    disable_irq(th->irq);
>>
>>      for (src = 0, dst = 0; dst <= th->num_thdevs; src++, dst++) {
>>          for (; src < ARRAY_SIZE(intel_th_subdevices); src++) {
> 
> [...]
> 
>> @@ -750,16 +752,19 @@ int intel_th_output_enable(struct intel_th *th, 
>> unsigned int otype)
>>              goto found;
>>      }
>>
>> +nodev:
>> +    enable_irq(th->irq);
>>      return -ENODEV;
>>
>>  found:
>>      thdev = intel_th_subdevice_alloc(th, &intel_th_subdevices[src]);
>>      if (IS_ERR(thdev))
>> -        return PTR_ERR(thdev);
>> -
>> -    th->thdev[th->num_thdevs++] = thdev;
>> +        ret = PTR_ERR(thdev);
>> +    else
>> +        th->thdev[th->num_thdevs++] = thdev;
>>
>> -    return 0;
>> +    enable_irq(th->irq);
>> +    return ret;
>>  }
>>  EXPORT_SYMBOL_GPL(intel_th_output_enable);
> 
> This is indeed a possible fix, but I believe a little bit of
> serialization can be employed here.

I was thinking there was a better approach.  Given the situation if 
you'd like me to test a fix, I can do so,

> 
> Lastly, my apologies for tardiness.

No worries.

Best,
-DA

> 
> Thanks!
> -- 
> Alex
>