linux-kernel - Re: [PATCH 1/2] string: Add stracpy and stracpy

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Date:   Wed, 24 Jul 2019 10:08:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Yann Droneaud <ydroneaud@...eya.com>,
        David Laight <David.Laight@...lab.com>,
        Joe Perches <joe@...ches.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>, Stephen Kitt <steve@....org>,
        Kees Cook <keescook@...omium.org>,
        Nitin Gote <nitin.r.gote@...el.com>,
        "jannh@...gle.com" <jannh@...gle.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms

On Wed, Jul 24, 2019 at 6:09 AM Rasmus Villemoes
<linux@...musvillemoes.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_.

              Linus