linux-kernel - Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE

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Message-ID: <20240730160432.GQ33588@noisy.programming.kicks-ass.net>
Date: Tue, 30 Jul 2024 18:04:32 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>, Miguel Ojeda <ojeda@...nel.org>,
	Kees Cook <kees@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Matthew Maurer <mmaurer@...gle.com>, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

On Tue, Jul 30, 2024 at 08:19:13AM -0700, Sami Tolvanen wrote:
> On Tue, Jul 30, 2024 at 3:29 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Jul 30, 2024 at 09:40:11AM +0000, Alice Ryhl wrote:
> > > Introduce a Kconfig option for enabling the experimental option to
> > > normalize integer types. This ensures that integer types of the same
> > > size and signedness are considered compatible by the Control Flow
> > > Integrity sanitizer.
> > >
> > > This option exists for compatibility with Rust, as C and Rust do not
> > > have the same set of integer types. There are cases where C has two
> > > different integer types of the same size and alignment, but Rust only
> > > has one integer type of that size and alignment. When Rust calls into
> > > C functions using such types in their signature, this results in CFI
> > > failures.
> > >
> > > This patch introduces a dedicated option for this because it is
> > > undesirable to have CONFIG_RUST affect CC_FLAGS in this way.
> >
> > To be clear, any code compiled with this is incompatible with code
> > compiled without this, as the function signatures will differ, right?
> >
> > Specifically, it will map things like 'unsigned long long' and 'unsigned
> > long' -- which are both u64 on LP64 targets to the same 'type', right?
> >
> > I suppose it has been decided the security impact of this change is
> > minimal?
> 
> I looked into this last year, and integer normalization reduced the
> number of unique type hashes in the kernel by ~1%, which should be
> fine.

Right, I didn't actually expect it to matter much either, but it
would've made good Changelog content.