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

[Greg KH <gregkh@linuxfoundation.org>]

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?

> ---
>  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