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Subject: Re: [PATCH RFC v2 5/6] staging: r8188eu: add error handling of
 rtw_read32
To:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Phillip Potter <phil@philpotter.co.uk>
Cc:     Larry Finger <Larry.Finger@lwfinger.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        Michael Straube <straube.linux@gmail.com>,
        "open list:STAGING SUBSYSTEM" <linux-staging@lists.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <cover.1629642658.git.paskripkin@gmail.com>
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 <2424268.2DCLMNk6Lg@localhost.localdomain>
From:   Pavel Skripkin <paskripkin@gmail.com>
Message-ID: <2355365e-d75e-8d1b-aad8-6979b70f33ec@gmail.com>
Date:   Tue, 24 Aug 2021 09:40:18 +0300
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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@gmail.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*().
> 

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.

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.


If I am missing something, please, let me know :) v3 is on the way...


With regards,
Pavel Skripkin