Re: [PATCH] usb: gadget: f_fs: Fix NULL pointer dereference in ffs_epfile_async_io_complete()

[Selvarasu Ganesan <quic_selvaras@quicinc.com>]

On 2/23/2024 11:28 AM, Greg KH wrote:
> On Thu, Feb 22, 2024 at 09:48:09PM -0800, Selvarasu Ganesan wrote:
>> In scenarios of continuous and parallel usage of multiple FFS interfaces
>> and concurrent adb operations (e.g., adb root, adb reboot), there's a
>> chance that ffs_epfile_async_io_complete() might be processed after
>> ffs_epfile_release(). This could lead to a NULL pointer dereference of
>> ffs when accessing the ffs pointer in ffs_epfile_async_io_complete(), as
>> ffs is freed as part of ffs_epfile_release(). This epfile release is
>> part of file operation and is triggered when user space daemons restart
>> themselves or a reboot is initiated.
>>
>> Fix this issue by adding a NULL pointer check for ffs in
>> ffs_epfile_async_io_complete().
>>
>> [  9981.393115] Unable to handle kernel NULL pointer dereference at virtual address 00000000000001e0
>> [  9981.402854] Mem abort info:
>> ...
>> [  9981.532540] Hardware name: Qualcomm Technologies,
>> [  9981.540579] pstate: 204000c5 (nzCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [  9981.548438] pc : ffs_epfile_async_io_complete+0x38/0x4c
>> [  9981.554529] lr : usb_gadget_giveback_request+0x30/0xd0
>> ...
>> [  9981.645057] Call trace:
>> [  9981.648282]  ffs_epfile_async_io_complete+0x38/0x4c
>> [  9981.654004]  usb_gadget_giveback_request+0x30/0xd0
>> [  9981.659637]  dwc3_gadget_endpoint_trbs_complete+0x1a8/0x48c
>> [  9981.666074]  dwc3_process_event_entry+0x378/0x648
>> [  9981.671622]  dwc3_process_event_buf+0x6c/0x288
>> [  9981.676903]  dwc3_thread_interrupt+0x3c/0x68
>> [  9981.682003]  irq_thread_fn+0x2c/0x8c
>> [  9981.686388]  irq_thread+0x198/0x2ac
>> [  9981.690685]  kthread+0x154/0x218
>> [  9981.694717]  ret_from_fork+0x10/0x20
>>
>> Signed-off-by: Selvarasu Ganesan <quic_selvaras@quicinc.com>
> 
> What commit id does this fix?  Should it go to stable kernels?

Fixes: 2e4c7553cd6f9 ("usb: gadget: f_fs: add aio support"). Yes it's
required to propagate to stable kernel as well.
> 
>> ---
>>   drivers/usb/gadget/function/f_fs.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
>> index be3851cffb73..d8c8e88628f9 100644
>> --- a/drivers/usb/gadget/function/f_fs.c
>> +++ b/drivers/usb/gadget/function/f_fs.c
>> @@ -849,7 +849,9 @@ static void ffs_epfile_async_io_complete(struct usb_ep *_ep,
>>   	usb_ep_free_request(_ep, req);
>>   
>>   	INIT_WORK(&io_data->work, ffs_user_copy_worker);
>> -	queue_work(ffs->io_completion_wq, &io_data->work);
>> +
>> +	if (ffs && ffs->io_completion_wq)
>> +		queue_work(ffs->io_completion_wq, &io_data->work);
> 
> What happens if ffs->io_compleation_wq goes away right after you test
> it but before you call queue_work()?
> 
> Where is the locking here to prevent that?
> 
> thanks,
> 
> greg k-h

Hi Greg,

Thank you for your feedback. I understand your concern about the
potential race condition with ffs->io_completion_wq. I’m considering
introducing a lock to protect this section of the code, but I wanted to
get your opinion on this.
In the f_fs.c driver, there are pre-existing locks. Would it be suitable 
to utilize these locks, or do you suggest the creation of a new lock 
specifically for ffs->io_completion_wq? We anticipate a performance 
impact if we use the existing lock, as it might be held by different
threads. What are your thoughts on this?"

Here’s what the code might look like with a new lock:

static void ffs_epfile_async_io_complete(struct usb_ep *_ep,
                                          struct usb_request *req)
{
....
spin_lock(&ffs->new_lock);
if (ffs && ffs->io_completion_wq)
     queue_work(ffs->io_completion_wq, &io_data->work);
spin_unlock(&ffs->new_lock);
....
}



static void ffs_data_put(struct ffs_data *ffs) {
...
destroy_workqueue(ffs->io_completion_wq);
kfree(ffs->dev_name);
spin_lock(&ffs->new_lock);
kfree(ffs);
spin_unlock(&ffs->new_lock);
...
}

Thanks,
Selva