From: Kees Cook <keescook@chromium.org>
To: Siddhesh Poyarekar <siddhesh@gotplt.org>
Cc: linux-hardening@vger.kernel.org,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Arnd Bergmann <arnd@arndb.de>, Juergen Gross <jgross@suse.com>,
Boris Ostrovsky <boris.ostrovsky@oracle.com>,
Tom Rix <trix@redhat.com>, Miguel Ojeda <ojeda@kernel.org>,
linux-kernel@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH 2/4] fortify: Explicitly check bounds are compile-time constants
Date: Wed, 21 Sep 2022 20:46:47 -0700 [thread overview]
Message-ID: <202209212034.16D9025882@keescook> (raw)
In-Reply-To: <b8f5f716-3ee6-87fd-d0e2-b1c35c98e0b0@gotplt.org>
On Wed, Sep 21, 2022 at 07:48:44AM -0400, Siddhesh Poyarekar wrote:
> On 2022-09-20 15:22, Kees Cook wrote:
> > In preparation for replacing __builtin_object_size() with
> > __builtin_dynamic_object_size(), all the compile-time size checks need
> > to check that the bounds variables are, in fact, known at compile-time.
> > Enforce what was guaranteed with __bos(). In other words, since all uses
> > of __bos() were constant expressions, it was not required to test for
> > this. When these change to __bdos(), they _may_ be constant expressions,
> > and the checks are only valid when the prior condition holds. This
> > results in no binary differences.
> >
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > include/linux/fortify-string.h | 50 +++++++++++++++++++++-------------
> > 1 file changed, 31 insertions(+), 19 deletions(-)
> >
> > diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> > index ff879efe94ed..71c0a432c638 100644
> > --- a/include/linux/fortify-string.h
> > +++ b/include/linux/fortify-string.h
> > @@ -80,6 +80,12 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
> > #define POS __pass_object_size(1)
> > #define POS0 __pass_object_size(0)
> > +#define __compiletime_lessthan(bounds, length) ( \
> > + __builtin_constant_p(length) && \
> > + __builtin_constant_p(bounds) && \
> > + bounds < length \
> > +)
>
> So with the gcc ranger, the compiler has lately been quite successful at
> computing a constant `bounds < length` even though bounds and length are not
> constant. So perhaps this:
>
> #define __compiletime_lessthan (bounds, length) ( \
> __builtin_constant (bounds < length) && \
> bounds < length \
> )
>
> might succeed in a few more cases.
Oh, interesting! That's very cool -- I never considered tossing a full
expression into __bcp. Yeah, that seems to work just fine:
https://godbolt.org/z/xrchErEx1
--
Kees Cook
next prev parent reply other threads:[~2022-09-22 3:46 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-20 19:21 [PATCH 0/4] fortify: Use __builtin_dynamic_object_size() when available Kees Cook
2022-09-20 19:21 ` [PATCH 1/4] x86/entry: Work around Clang __bdos() bug Kees Cook
2022-09-21 0:07 ` Boris Ostrovsky
2022-09-20 19:22 ` [PATCH 2/4] fortify: Explicitly check bounds are compile-time constants Kees Cook
2022-09-21 11:48 ` Siddhesh Poyarekar
2022-09-22 3:46 ` Kees Cook [this message]
2022-09-20 19:22 ` [PATCH 3/4] fortify: Convert to struct vs member helpers Kees Cook
2022-09-20 19:22 ` [PATCH 4/4] fortify: Use __builtin_dynamic_object_size() when available Kees Cook
2022-09-21 11:24 ` Miguel Ojeda
2022-09-21 11:43 ` Siddhesh Poyarekar
2022-09-22 3:33 ` Kees Cook
2022-09-22 14:45 ` Siddhesh Poyarekar
2022-11-22 10:20 ` Siddhesh Poyarekar
2022-11-23 5:15 ` Kees Cook
2022-11-23 15:29 ` Siddhesh Poyarekar
2023-01-13 15:59 ` linux-next - bxnt buffer overflow in strnlen Niklas Cassel
2023-01-13 16:08 ` linux-next - bnxt " Niklas Cassel
2023-01-13 22:44 ` Kees Cook
2023-01-16 10:56 ` Niklas Cassel
2022-09-22 20:26 ` [PATCH 0/4] fortify: Use __builtin_dynamic_object_size() when available Siddhesh Poyarekar
2022-09-23 0:20 ` Kees Cook
2022-09-23 0:55 ` Siddhesh Poyarekar
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