linux-kernel - Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of rtw

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Date:   Tue, 24 Aug 2021 02:10:37 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Pavel Skripkin <paskripkin@...il.com>,
        Phillip Potter <phil@...lpotter.co.uk>
Cc:     Larry Finger <Larry.Finger@...inger.net>,
        Greg KH <gregkh@...uxfoundation.org>,
        Michael Straube <straube.linux@...il.com>,
        "open list:STAGING SUBSYSTEM" <linux-staging@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of rtw_read32

On Tuesday, August 24, 2021 1:33:46 AM CEST Phillip Potter wrote:
> On Sun, 22 Aug 2021 at 15:36, Pavel Skripkin <paskripkin@...il.com> 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*().

Regards,

Fabio
> 
> Regards,
> Phil
>