rust: support for shadow call stack sanitizer

Add all of the flags that are needed to support the shadow call stack
(SCS) sanitizer with Rust, and updates Kconfig to allow only
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. Using this flag requires rustc version 1.82.0
or greater on the targets used by Rust in the kernel. This restriction
is reflected in Kconfig.

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 dependency is placed on `select HAVE_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.

At the time of writing, rustc 1.82.0 only exists via the nightly release
channel. There is a chance that the -Zsanitizer=shadow-call-stack flag
will end up needing 1.83.0 instead, but I think it is small.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240829-shadow-call-stack-v7-1-2f62a4432abf@google.com
[ Fixed indentation using spaces. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 0f3cd7c3a436..6d9234216c69 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -172,7 +172,7 @@ config RISCV
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_RETHOOK if !XIP_KERNEL
 	select HAVE_RSEQ
-	select HAVE_RUST if 64BIT
+	select HAVE_RUST if RUSTC_SUPPORTS_RISCV
 	select HAVE_SAMPLE_FTRACE_DIRECT
 	select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
 	select HAVE_STACKPROTECTOR
@@ -202,6 +202,13 @@ config RISCV
 	select UACCESS_MEMCPY if !MMU
 	select ZONE_DMA32 if 64BIT
 
+config RUSTC_SUPPORTS_RISCV
+	def_bool y
+	depends on 64BIT
+	# Shadow call stack requires rustc version 1.82+ due to use of the
+	# -Zsanitizer=shadow-call-stack flag.
+	depends on !SHADOW_CALL_STACK || RUSTC_VERSION >= 108200
+
 config CLANG_SUPPORTS_DYNAMIC_FTRACE
 	def_bool CC_IS_CLANG
 	# https://github.com/ClangBuiltLinux/linux/issues/1817