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

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Message-ID: <201907251301.E1E32DCCCE@keescook>
Date:   Thu, 25 Jul 2019 13:03:30 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        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>,
        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 10:08:57AM -0700, Linus Torvalds wrote:
> 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_.

Actually, perhaps we should add this test to strscpy() too?

diff --git a/lib/string.c b/lib/string.c
index 461fb620f85f..0e0d7628ddc4 100644
--- 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

-- 
Kees Cook