Re: [PATCH v2] dmaengine: idxd: Fix use-after-free of idxd_wq

[Guixin Liu <kanie@linux.alibaba.com>]

在 2026/4/24 13:43, Guixin Liu 写道:
>
>
> 在 2026/4/24 02:54, Vinicius Costa Gomes 写道:
>> Guixin Liu <kanie@linux.alibaba.com> writes:
>>
>>> 在 2026/4/23 05:34, Vinicius Costa Gomes 写道:
>>>> On Wed, 15 Apr 2026 17:50:30 +0800, Guixin Liu 
>>>> <kanie@linux.alibaba.com> wrote:
>>>>> diff --git a/drivers/dma/idxd/init.c b/drivers/dma/idxd/init.c
>>>>> index fb80803d5b57..c3cfd96074c9 100644
>>>>> --- a/drivers/dma/idxd/init.c
>>>>> +++ b/drivers/dma/idxd/init.c
>>>>> @@ -1293,13 +1293,30 @@ static void idxd_remove(struct pci_dev *pdev)
>>>>> [ ... skip 14 lines ... ]
>>>>> +     *
>>>>> +     * Use device_release_driver() to only unbind the driver 
>>>>> (triggering
>>>>> +     * idxd_device_drv_remove()) without touching sysfs. Then safely
>>>>> +     * unregister children before the parent.
>>>>> +     */
>>>>> +    device_release_driver(idxd_confdev(idxd));
>>> Thanks for the reply.
>>>> Below are sashiko comments verbatim, some notes:
>>>>
>>>>    - I do believe the race condition that it points out is real (but
>>>>      very narrow), but not sure that we don't have a similar one in
>>>>      current code (i.e. it wasn't introduced by your patch);
>>>>    - The possible wq leak was definitely not introduced by this patch;
>>>>    - The question about what happens with the file references after a
>>>>      device is removed is a separate issue;
>>>>
>>>> The patch looks good, not opposed to it at all, the only one I am a
>>>> bit concerned, about this patch, to be sure, is the race condition,
>>>> and the two "paths" to reach it. (the others we can fix separately)
>>>>
>>>> sashiko.dev <sashiko@sashiko.dev>:
>>>>
>>>> Does this introduce a race condition with sysfs?
>>>>
>>>> Since the driver's sysfs nodes remain active after 
>>>> device_release_driver()
>>>> is called, a user with sysfs privileges could write to the bind 
>>>> attribute
>>>> to re-probe the driver before the device unregistration completes.
>>>>
>>>> If idxd_device_drv_probe()->idxd_wqs_setup() runs concurrently with
>>>> idxd_unregister_devices(), does it cause a use-after-free when it
>>>> iterates over the idxd->wqs[] objects being freed, since
>>>> suppress_bind_attrs is not set?
>>> I think re-probe will call idxd_pci_probe() to allocate an new idxd 
>>> device,
>>> this avoids accessing stale data.
>> Yeah, I missed the (now obvious) that a new 'probe' also means a new
>> device. So this "path" is not valid. Thank you.
>>
>>>> Also, does this code leak the workqueue?
>>>>
>>>> The workqueue idxd->wq is dynamically allocated using 
>>>> create_workqueue()
>>>> during device initialization, but destroy_workqueue(idxd->wq) does not
>>>> appear to be called in idxd_remove() or the sysfs device release 
>>>> callback.
>>> Looks like it's b7cb9a034305 ("dmaengine: idxd: Fix refcount underflow
>>> on module unload")
>>> overlooked this, destory_workqueue(idxd->wq) is called in 
>>> idxd_cleanup()
>>> -> idxd_cleanup_internals().
>>>
>>> If so, I can send another patch to fix this.
>> Yes, please do.
> Will be added in v3.
Well, the idxd->wq is freed in idxd_conf_device_release(), so there is no
problem, no need to fix.
>>>>> idxd_unregister_devices(idxd);
>>>> sashiko.dev <sashiko@sashiko.dev>:
>>>>
>>>> If idxd_unregister_devices() drops the last reference to the child wq
>>>> objects and frees them, can idxd_shutdown() trigger a use-after-free
>>>> when called shortly after?
>>>>
>>>> For example, if a hardware error interrupt fired during teardown,
>>>> idxd_shutdown()->flush_workqueue() could execute an error handler that
>>>> iterates over the freed idxd->wqs[] memory.
>>>>
>>>> Additionally, if the hardware is wedged,
>>>> idxd_shutdown()->idxd_device_disable()->idxd_device_clear_state()
>>>> iterates over idxd->wqs[] and attempts to acquire 
>>>> mutex_lock(&wq->wq_lock)
>>>> on the freed memory.
>>> I took a look — idxd_shutdown() is only invoked by idxd_remove() and
>>> during system shutdown/reboot.
>>> Is idxd_shutdown() ever reached from the idxd hardware error interrupt
>>> path?
>> The bot's suggestion that this code path could be hit by a hardware
>> error is false, but the shutdown path (like the user pressing a button)
>> could be valid.
>>
>> A crash during that could cause the machine to not poweroff, which is
>> not good. Worth thinking about this, if there's something we could do
>> while we are here.
>
> Both idxd_remove() and idxd_shutdown() are called under 
> device_lock(dev) —
> the lock is acquired by the driver core before invoking any bus callback:
>
> 1.
>
>    Remove path: device_release_driver_internal() →
>    __device_driver_lock(dev, parent) → device_lock(dev) → ... →
>    pci_device_remove() → idxd_remove()
>
> 2.
>
>    Shutdown path: device_shutdown() → device_lock(dev) →
>    dev->bus->shutdown(dev) → pci_device_shutdown() → idxd_shutdown()
>
> Since both paths acquire the same dev->mutex on the same PCI device,
>
> idxd_remove() and idxd_shutdown() are mutually exclusive.
>
> Best Regards,
>
> Guixin Liu
>
>>>> Is there also a missing reference count for the cdev open path?
>>>>
>>>> Since idxd_unregister_devices() frees the wq structure, if user space
>>>> holds an open file descriptor, cdev_device_del() won't revoke it.
>>>> Can file operations like mmap or poll subsequently access the freed
>>>> ctx->wq pointer? Should idxd_cdev_open() take a kobject reference on
>>>> the parent wq device?
>>> To be honest, I'm not very familiar with the idxd driver. At this 
>>> point,
>>> it looks like the idxd driver needs a state machine to ensure mutual
>>> exclusion
>>> across the various concurrent paths.
>> Don't worry about this one, it's a separate issue.
>>
>>> Best Regards,
>>> Guixin Liu
>>>> via: 
>>>> https://sashiko.dev/#/message/20260415095030.42183-1-kanie@linux.alibaba.com
>>>> via: 
>>>> https://sashiko.dev/#/message/20260415095030.42183-1-kanie@linux.alibaba.com
>>>>
>>>> via: 
>>>> https://sashiko.dev/#/message/20260415095030.42183-1-kanie@linux.alibaba.com
>>>> via: 
>>>> https://sashiko.dev/#/message/20260415095030.42183-1-kanie@linux.alibaba.com
>>>>
>>
>> Cheers,
>