Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 21 Sep 2022 20:33:52 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Siddhesh Poyarekar <siddhesh@gotplt.org>
Cc:     linux-hardening@vger.kernel.org, Miguel Ojeda <ojeda@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, llvm@lists.linux.dev,
        Juergen Gross <jgross@suse.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/4] fortify: Use __builtin_dynamic_object_size() when
 available
Message-ID: <202209212032.5F392F42@keescook>
References: <20220920192202.190793-1-keescook@chromium.org>
 <20220920192202.190793-5-keescook@chromium.org>
 <9d80e0a3-3fcc-0676-4529-79743f418557@gotplt.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <9d80e0a3-3fcc-0676-4529-79743f418557@gotplt.org>
Precedence: bulk

On Wed, Sep 21, 2022 at 07:43:17AM -0400, Siddhesh Poyarekar wrote:
> On 2022-09-20 15:22, Kees Cook wrote:
> > Since the commits starting with c37495d6254c ("slab: add __alloc_size
> > attributes for better bounds checking"), the compilers have runtime
> > allocation size hints available in some places. This was immediately
> > available to CONFIG_UBSAN_BOUNDS, but CONFIG_FORTIFY_SOURCE needed
> > updating to explicitly make use the hints via the associated
> > __builtin_dynamic_object_size() helper. Detect and use the builtin when
> > it is available, increasing the accuracy of the mitigation. When runtime
> > sizes are not available, __builtin_dynamic_object_size() falls back to
> > __builtin_object_size(), leaving the existing bounds checking unchanged.
> 
> I don't know yet what the overhead is for __builtin_dynamic_object_size vs
> __builtin_object_size, were you able to measure it somehow for the kernel?
> If there's a significant tradeoff, it may make sense to provide a user
> override.

So far I've not seen any measurable performance difference, but I just
may not be creative enough yet.

So far, the tunable is building a kernel with or without FORTIFY_SOURCE
and UBSAN_BOUNDS. :)

-- 
Kees Cook