linux-hardening - Re: [PATCH v2] drivers/iio: Remove all strcpy() uses in favor of strscpy()

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Message-ID: <20210809102131.000021eb@Huawei.com>
Date:   Mon, 9 Aug 2021 10:21:31 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
CC:     Jonathan Cameron <jic23@...nel.org>, Len Baker <len.baker@....com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Kees Cook <keescook@...omium.org>,
        <linux-hardening@...r.kernel.org>,
        linux-iio <linux-iio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] drivers/iio: Remove all strcpy() uses in favor of
 strscpy()

On Sun, 8 Aug 2021 22:00:34 +0300
Andy Shevchenko <andy.shevchenko@...il.com> wrote:

> On Sun, Aug 8, 2021 at 7:25 PM Jonathan Cameron <jic23@...nel.org> wrote:
> >
> > On Sat,  7 Aug 2021 17:22:25 +0200
> > Len Baker <len.baker@....com> wrote:
> >  
> > > strcpy() performs no bounds checking on the destination buffer. This
> > > could result in linear overflows beyond the end of the buffer, leading
> > > to all kinds of misbehaviors. The safe replacement is strscpy().
> > >
> > > This patch is an effort to clean up the proliferation of str*()
> > > functions in the kernel and a previous step in the path to remove
> > > the strcpy function from the kernel entirely [1].
> > >
> > > [1] https://github.com/KSPP/linux/issues/88
> > >
> > > Signed-off-by: Len Baker <len.baker@....com>  
> > Applied to the togreg branch of iio.git and pushed out as testing
> > so 0-day can poke at it and see if we missed anything.  
> 
> Isn't it too early? Or am I missing something (see below)?
> 
> ...
> 
> > >                       /* use length + 2 for adding minus sign if needed */
> > > -                     str = devm_kzalloc(regmap_get_device(st->map),
> > > -                                        strlen(orient) + 2, GFP_KERNEL);
> > > +                     n = strlen(orient) + 2;
> > > +                     str = devm_kzalloc(regmap_get_device(st->map), n,
> > > +                                        GFP_KERNEL);
> > >                       if (str == NULL)
> > >                               return -ENOMEM;
> > >                       if (strcmp(orient, "0") == 0) {
> > > -                             strcpy(str, orient);
> > > +                             strscpy(str, orient, n);
> > >                       } else if (orient[0] == '-') {
> > > -                             strcpy(str, &orient[1]);
> > > +                             strscpy(str, &orient[1], n);
> > >                       } else {
> > >                               str[0] = '-';
> > > -                             strcpy(&str[1], orient);
> > > +                             strscpy(&str[1], orient, n - 1);  
> 
> Why n-1?

n is the total length and this is printing from [1], so n - 1 is remaining
space.

> 
> > >                       }  
> 
> As far as I understood the logic, it  inverts the sign except the case
> when it equals 0.
> 
> I have a question here, why can't we always use +/-?
> Why can't 0 be prefixed with a sign?

Technically a userspace ABI change, but I agree it should be fairly
harmless unless someone is rolling their own string handling routines.

> 
> If the above can be used, we may simplify this code.
> 
> Len, I think this task may be considered simple, but I recommend
> thinking about each case and finding a way to simplify it more.
> 

It could be a little simpler doing this, so 'maybe' worth doing.

Jonathan