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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
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