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Message-ID: <ffcdd323f2f325fc9e9f2e17e1795ddaddeccd33.camel@perches.com>
Subject: Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
From:   Joe Perches <joe@perches.com>
To:     Kees Cook <keescook@chromium.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Yann Droneaud <ydroneaud@opteya.com>,
        David Laight <David.Laight@aculab.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>, Stephen Kitt <steve@sk2.org>,
        Nitin Gote <nitin.r.gote@intel.com>,
        "jannh@google.com" <jannh@google.com>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
        Andrew Morton <akpm@linux-foundation.org>
Date:   Thu, 25 Jul 2019 19:46:53 -0700
In-Reply-To: <201907251301.E1E32DCCCE@keescook>
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On Thu, 2019-07-25 at 13:03 -0700, Kees Cook wrote:
> On Wed, Jul 24, 2019 at 10:08:57AM -0700, Linus Torvalds wrote:
> > On Wed, Jul 24, 2019 at 6:09 AM Rasmus Villemoes
> > <linux@rasmusvillemoes.dk> wrote:
> > > The kernel's snprintf() does not behave in a non-standard way, at least
> > > not with respect to its return value.
> > 
> > Note that the kernels snprintf() *does* very much protect against the
> > overflow case - not by changing the return value, but simply by having
> > 
> >         /* Reject out-of-range values early.  Large positive sizes are
> >            used for unknown buffer sizes. */
> >         if (WARN_ON_ONCE(size > INT_MAX))
> >                 return 0;
> > 
> > at the very top.
> > 
> > So you can't actually overflow in the kernel by using the repeated
> > 
> >         offset += vsnprintf( .. size - offset ..);
> > 
> > model.
> > 
> > Yes, it's the wrong thing to do, but it is still _safe_.
> 
> Actually, perhaps we should add this test to strscpy() too?

Doesn't seem to have a reason not to be added
but maybe it's better to add another WARN_ON_ONCE.

> diff --git a/lib/string.c b/lib/string.c
[]
> @@ -182,7 +182,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
>  	size_t max = count;
>  	long res = 0;
>  
> -	if (count == 0)
> +	if (count == 0 || count > INT_MAX)
>  		return -E2BIG;
>  
>  #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>