Re: [PATCH 1/2] cfi: add CONFIG_CFI_ICALL_NORMALIZE_INTEGERS

[Sami Tolvanen]

On Tue, Jul 30, 2024 at 3:29 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.

Sami