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Date: Thu, 25 Jul 2019 19:46:53 -0700
From: Joe Perches <joe@...ches.com>
To: Kees Cook <keescook@...omium.org>, Linus Torvalds
<torvalds@...ux-foundation.org>
Cc: Rasmus Villemoes <linux@...musvillemoes.dk>, Yann Droneaud
<ydroneaud@...eya.com>, David Laight <David.Laight@...lab.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 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@...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?
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
>
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