linux-kernel - Re: [PATCH RFC 0/3] staging: r8188eu: avoid uninit value bugs

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Date:   Sun, 22 Aug 2021 12:59:13 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Larry.Finger@...inger.net, phil@...lpotter.co.uk,
        gregkh@...uxfoundation.org, straube.linux@...il.com,
        Pavel Skripkin <paskripkin@...il.com>
Cc:     linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Martin Kaiser <martin@...ser.cx>
Subject: Re: [PATCH RFC 0/3] staging: r8188eu: avoid uninit value bugs

On Sunday, August 22, 2021 12:09:29 PM CEST Pavel Skripkin wrote:
> On 8/22/21 12:53 PM, Fabio M. De Francesco wrote:
> > On Friday, August 20, 2021 7:07:28 PM CEST Pavel Skripkin wrote:
> >> Hi, Greg, Larry and Phillip!
> >> 
> >> I noticed, that new staging driver was added like 3 weeks ago and I 
decided
> >> to look at the code, because drivers in staging directory are always 
buggy.
> >> 
> >> The first thing I noticed is *no one* was checking read operations 
result,
> > 
> > but
> > 
> >> it can fail and driver may start writing random stack values into 
registers.
> > 
> > It
> > 
> >> can cause driver misbehavior or device misbehavior.
> > 
> > After the messages I wrote yesterday, I had some minutes to look deeper at 
the
> > code that would be changed by these patches.
> > 
> > I think that it does not look like that the driver could return "random 
stack
> > values into registers" and I think this entire series in unnecessary.
> > 
> > As far as I understand this driver (though I must admit that I really 
don't
> > know how to write drivers, and I'm not interested in understanding - at 
the
> > moment, at least), all the usb_read*() call usbctrl_vendorreq() and the 
latter
> > *does* proper error checking before returning to the callers the read 
data.
> > 
> > Please, look at the code copied from usbctrl_vendorreq() and pasted here 
(some
> > comments are mine):
> > 
> > /* start of code */
> > static int usbctrl_vendorreq(struct intf_hdl *pintfhdl, u16 value, void
> > *pdata, u16 len, u8 requesttype)
> > {
> > 
> > /* test if everything is OK for transfers and setup the necessary 
variables */
> > [...]
> > 
> > status = usb_control_msg(udev, pipe, REALTEK_USB_VENQT_CMD_REQ,
> > 
> >                                           reqtype, value,
> > 
> > REALTEK_USB_VENQT_CMD_IDX,
> > 
> >                                           pIo_buf, len,
> > 
> > RTW_USB_CONTROL_MSG_TIMEOUT);
> > 
> >                  if (status == len) {   /*  Success this control transfer. 
*/
> >                  
> >                          rtw_reset_continual_urb_error(dvobjpriv);
> >                          if (requesttype == 0x01)
> >                          
> >                                  memcpy(pdata, pIo_buf,  len); /* pdata
> > 
> > receives the read data */
> > 
> > 	} else { /*  error cases */
> > 
> > [...]
> > 
> > }
> > /* end of code */
> > 
> > So, *I cannot ack this RFC*, unless maintainers say I'm missing something.
> > 
> > Larry, Philip, since you have much more knowledge than me about r8188eu 
(and,
> > more in general, on device drivers) may you please say what you think 
about my
> > arguments against this series?
> 
> Hi, Fabio!
> 
> Thank you for looking into this, but I still can see the case when pdata
> won't be initialized:
> 
> 
> pdata is initialized only in case of successful transfer, i.e len > 0.
> It means some data was received (maybe not full length, but anyway). In
> case of usb_control_msg() error (for example -ENOMEM) code only does
> this code block:
> 
> if (status < 0) {
> 	if (status == (-ESHUTDOWN) || status == -ENODEV) {
> 		adapt->bSurpriseRemoved = true;
> 	} else {
> 		struct hal_data_8188e	*haldata = GET_HAL_DATA(adapt);
> 		haldata->srestpriv.Wifi_Error_Status = 
USB_VEN_REQ_CMD_FAIL;
> 	}
> }

It's up to the callers of _rtw_usb*() to check return values and then act 
accordingly. 

It doesn't matter whether or not *pdata is initialized because usb_read*() 
returns data = 0 if usb_control_msg() has not initialized/changed  its third 
parameter. Then _rtw_read*() receive 0 or initialized data depending on errors 
or no errors. Finally _rtw_read*() returns that same value to the callers (via 
r_val). 

So, it's up to the callers to test if (!_rtw_read*()) and then act 
accordingly. If they get 0 they should know how to handle the errors.

Furthermore, we have already either adapt->bSurpriseRemoved = true or haldata-
>srestpriv.Wifi_Error_Status = USB_VEN_REQ_CMD_FAIL. Depending on contexts 
where _rtw_read*() are called, perhaps they could also check the two variables 
above.

In summation. if anything should be changed, it is the code of the callers of 
_rtw_read*() if you find out they they don't properly handle the returning 
values of this function. You should find every place where _rtw_read*() are 
called and figure out if the returns are properly checked and handled; if not, 
make some change only there.

Larry, Philip, where are you? Am I missing something?

Thanks,

Fabio

> 
> And then just loops further. In case of 10 ENOMEM in a row,. passed
> pdata won't be initialized at all and driver doesn't do anything about
> it. I believe, it's not good approach to play with random values. We
> should somehow handle transfer errors all across the driver.
> 
> If I am missing something, please, let me know :)
> 
> 
> 
> With regards,
> Pavel Skripkin