linux-kernel - Re: [PATCH 1/1] drivers/usb/chipidea/debuc.c: avoid out of bound read

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Message-ID: <20150428082222.GB32234@shlinux2>
Date:	Tue, 28 Apr 2015 16:22:24 +0800
From:	Peter Chen <peter.chen@...escale.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] drivers/usb/chipidea/debuc.c: avoid out of bound read

On Mon, Apr 27, 2015 at 08:25:12PM +0200, Heinrich Schuchardt wrote:
> On 22.04.2015 03:26, Peter Chen wrote:
> > On Tue, Apr 21, 2015 at 09:25:41PM +0200, Heinrich Schuchardt wrote:
> >> Hello Peter,
> >>
> >> thanks for reviewing.
> >>
> >> On 21.04.2015 03:32, Peter Chen wrote:
> >>> On Fri, Apr 17, 2015 at 08:04:13AM +0200, Heinrich Schuchardt wrote:
> >>>> A string written by the user may not be zero terminated.
> >>>>
> >>>> sscanf may read memory beyond the buffer if no zero byte
> >>>> is found.
> >>>>
> >>>> Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
> >>>> ---
> >>>>  drivers/usb/chipidea/debug.c | 6 +++++-
> >>>>  1 file changed, 5 insertions(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/usb/chipidea/debug.c b/drivers/usb/chipidea/debug.c
> >>>> index dfb05ed..ef08af3 100644
> >>>> --- a/drivers/usb/chipidea/debug.c
> >>>> +++ b/drivers/usb/chipidea/debug.c
> >>>> @@ -88,9 +88,13 @@ static ssize_t ci_port_test_write(struct file *file, const char __user *ubuf,
> >>>>  	char buf[32];
> >>>>  	int ret;
> >>>>  
> >>>> -	if (copy_from_user(buf, ubuf, min_t(size_t, sizeof(buf) - 1, count)))
> >>>> +	count = min_t(size_t, sizeof(buf) - 1, count);
> >>>> +	if (copy_from_user(buf, ubuf, count))
> >>>>  		return -EFAULT;
> >>>
> >>> Any reasons to change above?
> >>
> >> Otherwise we would have two lines with the term
> >> min_t(size_t, sizeof(buf) - 1, count).
> > 
> > Sorry, two lines of min_t(..)? Why I can't find it?
> 
> Hello Peter,
> 
> in my patch I write:
> 
> count = min_t(size_t, sizeof(buf) - 1, count);
> if (copy_from_user(buf, ubuf, count))
> 	return -EFAULT;
> 
> /* sscanf requires a zero terminated string */
> buf[count] = 0;
> 
> Without the first part of the change I would have had to write:
> 
> if (copy_from_user(buf, ubuf, min_t(size_t, sizeof(buf) - 1, count)))
> 	return -EFAULT;
> 
> /* sscanf requires a zero terminated string */
> buf[min_t(size_t, sizeof(buf) - 1, count)] = 0;
> 
> We should do the same calculation
> "min_t(size_t, sizeof(buf) - 1, count)"
> only once in the coding.

Oh, yeah. 
Send your v2 with '\0' change, thanks.

Peter

> 
> Best regards
> 
> Heinrich
> 
> 
> > 
> > 
> >>
> >> This would make the code harder to read.
> >>
> >>>>  
> >>>> +	/* sscanf requires a zero terminated string */
> >>>> +	buf[count] = 0;
> >>>> +
> >>>
> >>> I prefer using '\0'
> >>
> >> If you confirm the rest of the patch is ok, I will send an updated patch.
> >>
> >> Best regards
> >>
> >> Heinrich
> >>
> >>>
> >>>>  	if (sscanf(buf, "%u", &mode) != 1)
> >>>>  		return -EINVAL;
> >>>>  
> >>
> > 
> 

-- 

Best Regards,
Peter Chen
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