Re: [PATCH 2/7] s390/pci: fix use-after-free race in zpci floating interrupt cleanup

["Tobias Schumacher" <ts@linux.ibm.com>]

On Wed Aug 19, 2026 at 11:20 AM CEST, sashiko-bot wrote:
-- snip --

>> diff --git a/arch/s390/pci/pci_irq.c b/arch/s390/pci/pci_irq.c
>> index c9520a16ca753..94b03d16006ba 100644
>> --- a/arch/s390/pci/pci_irq.c
>> +++ b/arch/s390/pci/pci_irq.c
>> @@ -278,8 +278,14 @@ static void zpci_floating_irq_handler(struct airq_struct *airq,
>>  			continue;
>>  		}
>>
>> +		rcu_read_lock();
>> +
>>  		/* Scan the adapter interrupt vector for this device. */
>> -		aibv = zpci_ibv[si];
>> +		aibv = rcu_dereference(zpci_ibv[si]);
>
> [Severity: Medium]
> I see that rcu_dereference() is used here to read from the array.
>
> Does the write side in __alloc_airq() also need to be updated to use
> rcu_assign_pointer()?
>
> arch/s390/pci/pci_irq.c:__alloc_airq() {
>     ...
> 		/* Wire up shortcut pointer */
> 		zpci_ibv[*bit] = zdev->aibv;
>     ...
> }
>
> Without rcu_assign_pointer(), the required memory release barrier is missing
> when the interrupt vector is published to the RCU-protected array.

Yes, will fix that.

>> +		if (!aibv) {
>> +			rcu_read_unlock();
>> +			continue;
>> +		}
>
> [ ... ]
>
>> @@ -346,9 +353,12 @@ static void zpci_msi_teardown_directed(struct zpci_dev *zdev)
>>
>>  static void zpci_msi_teardown_floating(struct zpci_dev *zdev)
>>  {
>> +	airq_iv_free_bit(zpci_sbv, zdev->aisb);
>> +	zpci_ibv[zdev->aisb] = NULL;
>> +	synchronize_rcu();
>
> [Severity: High]
> Does freeing the bit before clearing the array pointer create a race condition?
>
> If Thread A (teardown) frees the bit, Thread B (initializing a new device)
> could allocate the exact same bit from zpci_sbv. Thread B would then allocate
> its vector and assign it to zpci_ibv[bit]. When Thread A continues, it clears
> zpci_ibv[zdev->aisb], overwriting Thread B's valid pointer with NULL.
>
> Could this overwrite lead to permanently lost interrupts and a memory leak for
> the new device?

Looks like you're right, I'll investigate and fix it in a follow-up
series.

Tobias