Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang

From: Kees Cook <keescook@chromium.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: George Burgess IV <gbiv@google.com>,
	Miguel Ojeda <ojeda@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org,
	Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH v6 1/4] Compiler Attributes: Add __pass_object_size for Clang
Date: Thu, 3 Feb 2022 16:29:22 -0800	[thread overview]
Message-ID: <202202031628.E28BBBB490@keescook> (raw)
In-Reply-To: <CAKwvOdnUHFi=1+9VxsfwxNAkTFEytg5FV=Aii7awSx+ioRaQbQ@mail.gmail.com>

On Thu, Feb 03, 2022 at 02:01:06PM -0800, Nick Desaulniers wrote:
> On Thu, Feb 3, 2022 at 12:58 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Feb 03, 2022 at 12:18:24PM -0800, Nick Desaulniers wrote:
> > > On Thu, Feb 3, 2022 at 9:33 AM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > In order to gain greater visibility to type information when using
> > > > __builtin_object_size(), Clang has a function attribute "pass_object_size"
> > > > that will make size information available for marked arguments in
> > > > a function by way of implicit additional function arguments that are
> > > > then wired up the __builtin_object_size().
> > > >
> > > > This is needed to implement FORTIFY_SOURCE in Clang, as a workaround
> > > > to Clang's __builtin_object_size() having limited visibility[1] into types
> > > > across function calls (even inlines).
> > > >
> > > > Since any usage must also be const, include it in the macro.
> > >
> > > I think it might be helpful to quote the docs
> > > (https://clang.llvm.org/docs/AttributeReference.html#pass-object-size-pass-dynamic-object-size)
> > >
> > > >> Additionally, any parameter that pass_object_size is applied to must be marked const at its function’s definition.
> > >
> > > One thing that's concerning to me is though:
> > >
> > > >> It is an error to take the address of a function with pass_object_size on any of its parameters.
> > >
> > > Surely the kernel has indirect calls to some of these functions
> > > somewhere? Is that just an issue for C++ name-mangling perhaps?
> >
> > AFAIU, this shouldn't be a problem for any of these. Nothing is trying
> > to take memcpy, memset, etc by address. The macro-ified version of this
> > change proved that out. :)
> 
> I thought Sami had found a location where memcpy was invoked
> indirectly as part of his kcfi work? Maybe I'm misremembering.
> 
> https://github.com/samitolvanen/linux/commit/46a777fb35784a8c6daf13d67de8bfb5148adc2a#diff-a27660992abdf360d01deac6364db31836d0d98b5d9573b7fc10a6785a669975R16

Hm, I've had memcpy as a macro for a while now, so dunno! That's not a
sensible thing to call indirectly. :)

-- 
Kees Cook