Re: [PATCH v3 1/3] staging: rtl8712: fix uninit-value in usb_read8() and friends

[Wang Cheng]

On 22/05/06 03:12PM, Pavel Skripkin wrote:
> Hi Wang,
> 
> On 5/6/22 14:59, Wang Cheng wrote:
> > Reported-and-tested-by: syzbot+6f5ecd144854c0d8580b@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Wang Cheng <wanngchenng@xxxxxxxxx>
> > ---
> >   drivers/staging/rtl8712/usb_ops.c | 27 ++++++++++++++++++---------
> >   1 file changed, 18 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/staging/rtl8712/usb_ops.c b/drivers/staging/rtl8712/usb_ops.c
> > index e64845e6adf3..af9966d03979 100644
> > --- a/drivers/staging/rtl8712/usb_ops.c
> > +++ b/drivers/staging/rtl8712/usb_ops.c
> > @@ -29,7 +29,8 @@ static u8 usb_read8(struct intf_hdl *intfhdl, u32 addr)
> >   	u16 wvalue;
> >   	u16 index;
> >   	u16 len;
> > -	__le32 data;
> > +	int status;
> > +	__le32 data = 0;
> >   	struct intf_priv *intfpriv = intfhdl->pintfpriv;
> >   	request = 0x05;
> > @@ -37,8 +38,10 @@ static u8 usb_read8(struct intf_hdl *intfhdl, u32 addr)
> >   	index = 0;
> >   	wvalue = (u16)(addr & 0x0000ffff);
> >   	len = 1;
> > -	r8712_usbctrl_vendorreq(intfpriv, request, wvalue, index, &data, len,
> > -				requesttype);
> > +	status = r8712_usbctrl_vendorreq(intfpriv, request, wvalue, index,
> > +					 &data, len, requesttype);
> > +	if (status < 0)
> > +		return 0;
> 
> Wait, but what about partial reads? I see that r8712_usbctrl_vendorreq()
> uses `usb_control_msg()` which can read less data than was requested.
> 
> So here you are just hiding these error by zeroing data
> 
> >   	return (u8)(le32_to_cpu(data) & 0x0ff);
> >   }
> 
> And as i said in previous email: caller can't see an error.
> 
> Imagine caller waits for some register to become 0. Caller sees that
> usb_read8() returned 0. Is it an error? Is register really become 0?
> 
> 
> Generic read API prototype looks like following:
> 
> int read_something(struct my_cool_dev *dev, void *data, size_t size);
> 
> and it returns 0 on success and -errno on failure and data returned via
> passed pointer. So, if API returned an error caller should not touch @data,
> since likely it's uninitialized

Thank you for your remind, sorry I didnt understand your previous reply
about "register and 0" that you explain very clearly above. I am thinking
about it standing on that the return type of usb_read8() is u8.

thanks,
- w