Re: [PATCH v5] rust: support for shadow call stack sanitizer

[Kees Cook <kees@kernel.org>]

On Tue, Aug 06, 2024 at 10:01:44AM +0000, Alice Ryhl wrote:
> This patch adds all of the flags that are needed to support the shadow
> call stack (SCS) sanitizer with Rust, and updates Kconfig to allow
> configurations that work.
> 
> The -Zfixed-x18 flag is required to use SCS on arm64, and requires rustc
> version 1.80.0 or greater. This restriction is reflected in Kconfig.
> 
> When CONFIG_DYNAMIC_SCS is enabled, the build will be configured to
> include unwind tables in the build artifacts. Dynamic SCS uses the
> unwind tables at boot to find all places that need to be patched. The
> -Cforce-unwind-tables=y flag ensures that unwind tables are available
> for Rust code.
> 
> In non-dynamic mode, the -Zsanitizer=shadow-call-stack flag is what
> enables the SCS sanitizer.
> 
> At the time of writing, all released rustc versions up to and including
> 1.81 incorrectly think that the Rust targets aarch64-unknown-none and
> riscv64-unknown-none-elf don't support -Zsanitizer=shadow-call-stack, so
> the build will fail if you enable shadow call stack in non-dynamic mode.
> See [1] for the relevant feature request. To avoid this compilation
> failure, Kconfig is set up to reject such configurations.
> 
> Note that because these configurations are rejected, this patch only
> allows SCS to be used with arm64 and not on riscv. However, once [1] is
> implemented, I will submit a follow-up patch that allows configurations
> without UNWIND_PATCH_PAC_INTO_SCS on sufficiently new compilers. That
> patch will implicitly allow SCS to be enabled on riscv, but this is okay
> because unlike arm64, riscv does not need any flags for rustc beyond
> -Zsanitizer=shadow-call-stack.
> 
> It is possible to avoid the requirement of rustc 1.80.0 by using
> -Ctarget-feature=+reserve-x18 instead of -Zfixed-x18. However, this flag
> emits a warning during the build, so this patch does not add support for
> using it and instead requires 1.80.0 or greater.
> 
> The `depends on` clause is placed on `config RUST` to avoid a situation
> where enabling Rust silently turns off the sanitizer. Instead, turning
> on the sanitizer results in Rust being disabled. We generally do not
> want changes to CONFIG_RUST to result in any mitigations being changed
> or turned off.
> 
> Link: https://github.com/rust-lang/rust/issues/121972 [1]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

I'd like to make sure Ard is happy with this, but from what I can see it
looks correct. Thanks!

Reviewed-by: Kees Cook <kees@kernel.org>

-- 
Kees Cook