linux-hardening - Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with strscpy

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Date:   Wed, 14 Jun 2023 10:51:01 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Azeem Shaikh <azeemshaikh38@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-hardening@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Albert Briscoe <albertsbriscoe@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>
Subject: Re: [PATCH] usb: gadget: function: printer: Replace strlcpy with
 strscpy

On Wed, Jun 14, 2023 at 10:12:14AM -0400, Azeem Shaikh wrote:
> On Tue, Jun 13, 2023 at 3:30 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Tue, Jun 13, 2023 at 12:43:41AM +0000, Azeem Shaikh wrote:
> [...]
> > > -     result = strlcpy(page, opts->pnp_string, PAGE_SIZE);
> > > -     if (result >= PAGE_SIZE) {
> > > +     result = strscpy(page, opts->pnp_string, PAGE_SIZE);
> > > +     if (result == -E2BIG) {
> >
> > I think "< 1" might be a better test here.
> 
> Curious, why "< 1" instead of "< 0"?
> 
> > >               result = PAGE_SIZE;
> > >       } else if (page[result - 1] != '\n' && result + 1 < PAGE_SIZE) {

It's for this case above where "result" may be used in an array index,
and if it's 0 or less, there will be a negative array index (due to the "-
1"). So, here, it needs to be "< 1" instead of the more traditional "< 0".

-- 
Kees Cook