Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of rtw_read32

[Pavel Skripkin]

On 8/24/21 11:38 AM, Fabio M. De Francesco wrote:
> On Tuesday, August 24, 2021 8:40:18 AM CEST Pavel Skripkin wrote:
> > On 8/24/21 3:10 AM, Fabio M. De Francesco wrote:
> > > On Tuesday, August 24, 2021 1:33:46 AM CEST Phillip Potter wrote:
> > >> On Sun, 22 Aug 2021 at 15:36, Pavel Skripkin <paskripkin@xxxxxxxxx> wrote:
> > >> > -static u32 usb_read32(struct intf_hdl *pintfhdl, u32 addr)
> > >> > +static int usb_read32(struct intf_hdl *pintfhdl, u32 addr, u32 *data)
> > >> >  {
> > >> >         u8 requesttype;
> > >> >         u16 wvalue;
> > >> >         u16 len;
> > >> > -       __le32 data;
> > >> > +       int res;
> > >> > +       __le32 tmp;
> > >> > +
> > >> > +       if (WARN_ON(unlikely(!data)))
> > >> > +               return -EINVAL;
> > >> >
> > >> >         requesttype = 0x01;/* read_in */
> > >> >
> > >> >         wvalue = (u16)(addr & 0x0000ffff);
> > >> >         len = 4;
> > >> >
> > >> > -       usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
> > >> > +       res = usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
> > >> > +       if (res < 0) {
> > >> > +               dev_err(dvobj_to_dev(pintfhdl->pintf_dev), "Failed to read 32 bytes: %d\n", res);
> > >> > +       } else {
> > >> > +               /* Noone cares about positive return value */
> > >> > +               *data = le32_to_cpu(tmp);
> > >> > +               res = 0;
> > >> > +       }
> > >> >
> > >> > -       return le32_to_cpu(data);
> > >> > +       return res;
> > >> >  }
> > >> 
> > >> Dear Pavel,
> > >> 
> > >> OK, found the issue with decoded stack trace after reviewing this
> > >> usb_read32 function. Your line:
> > >> res = usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);
> > >> 
> > >> should read:
> > >> res = usbctrl_vendorreq(pintfhdl, wvalue, &tmp, len, requesttype);
> > > 
> > > Dear Philip,
> > > 
> > > No, it should read:
> > > 
> > > res = usbctrl_vendorreq(pintfhdl, wvalue, data, len, requesttype);
> > > 
> > > I suspect that Pavel didn't notice he was reusing a line of the old code
> > > wth no due changes.
> > > 
> > >> With this change, the driver runs fine with no crashes/oopses. I will
> > >> explain the issue but you can probably see already, so I hope I'm not
> > >> coming across as patronising, just trying to be helpful :-)
> > >> 
> > >> Essentially, you are taking the address of the data function parameter
> > >> on this line with &data, a pointer to u32, which is giving you a
> > >> pointer to a pointer to u32 (u32 **) for this function parameter
> > >> variable. When passed to usbctrl_vendorreq, it is being passed to
> > >> memcpy inside this function as a void *, meaning that memcpy
> > >> subsequently overwrites the value of the memory address inside data to
> > >> point to a different location, which is problem when it is later
> > >> deferenced at:
> > >> *data = le32_to_cpu(tmp);
> > >> causing the OOPS
> > >> 
> > >> Also, as written, you can probably see that tmp is uninitialised. This
> > >> looks like a typo, so guessing this wasn't your intention. Anyhow,
> > >> with that small change, usbctrl_vendorreq reads into tmp, which is
> > >> then passed to le32_to_cpu whose return value is stored via the
> > >> deferenced data ptr (which now has its original address within and not
> > >> inadvertently modified). Hope this helps, and I'd be happy to Ack the
> > >> series if you want to resend this patch. Many thanks.
> > > 
> > > I think that another typo is having 'tmp', because that variable is unnecessary
> > > and "*data = le32_to_cpu(tmp);" is wrong too.
> > > 
> > > Now I also see that also usb_read16() is wrong, while usb_read8() (the one that
> > > I had read yesterday) is the only correct function of the three usb_read*().
> > > 
> >
> > Hi, guys!
> >
> >
> > Sorry for breaking your system, Phillip. This code was part of "last 
> > minute" changes and yes, it's broken :)
> >
> > I get what Phillip said, because I _should_ read into tmp variable 
> > instead of directly to data, but I don't get Fabio's idea, sorry.
>
> Hi Pavel,
>
> I (wrongly?) assumed from the prototype of usb_read32() that u32 *data is in native
> endianness. So, I didn't see the necessity of using _le32 tmp and then convert that tmp
> with le32_to_cpu().
>
> I simply thought that data could be passed to usbctrl_vendorreq as it-is.
>
> > Data from chip comes in little-endian, so we _should_ convert it to 
> > cpu's endian. Temp variable is needed to make smatch and all other 
> > static anylis tools happy about this code.
>
> Now that you explained that "Data from chip comes in little-endian", obviously
> I must agree with you that the code needs tmp and that tmp must be
> swapped by le32_to_cpu(), ahead of assigning it to *data.
>
> Just a curiosity... Since I was not able to see that *data is returned in little endian,
> can you please point me where in the code you found out that it is? There must
> be some place in the code that I'm unable to find and see that *data is LE.
>
> Thanks in advance,
>
> Fabio

Hi, Fabio!

previous usb_read16() realization, which is 100% right:


static u16 usb_read16(struct intf_hdl *pintfhdl, u32 addr)
{
	u8 requesttype;
	u16 wvalue;
	u16 len;
	__le32 data;

	requesttype = 0x01;/* read_in */
	wvalue = (u16)(addr & 0x0000ffff);
	len = 2;
	usbctrl_vendorreq(pintfhdl, wvalue, &data, len, requesttype);

	return (u16)(le32_to_cpu(data) & 0xffff);
}


Bases on this code, I think, it's oblivious, that data comes in 
little-endian. That's why I leaved temp variable for casting le32 to 
cpu's endianess.

I could just read into u{16,32} * and then make smth like

*data = le32_to_cpu(*data)

but static analysis tools will complain about wrong data type passed to 
 le32_to_cpu()

+ Phillip tested fixed v2 version and it worked well for him. I guess, 
Phillip was able to spot weird driver behavior, if this cast is wrong.




With regards,
Pavel Skripkin