rust: support for shadow call stack sanitizer

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: rust-lang/rust#121972 [1]
Signed-off-by: Alice Ryhl <[email protected]>

Author: Darksonn

Makefile

@@ -927,6 +927,7 @@ ifdef CONFIG_SHADOW_CALL_STACK
 ifndef CONFIG_DYNAMIC_SCS
 CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
 KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
+KBUILD_RUSTFLAGS += -Zsanitizer=shadow-call-stack
 endif
 export CC_FLAGS_SCS
 endif

arch/arm64/Makefile

@@ -57,9 +57,11 @@ KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
 ifneq ($(CONFIG_UNWIND_TABLES),y)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
 KBUILD_AFLAGS	+= -fno-asynchronous-unwind-tables -fno-unwind-tables
+KBUILD_RUSTFLAGS += -Cforce-unwind-tables=n
 else
 KBUILD_CFLAGS	+= -fasynchronous-unwind-tables
 KBUILD_AFLAGS	+= -fasynchronous-unwind-tables
+KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y -Zuse-sync-unwind=n
 endif
 
 ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
@@ -114,6 +116,7 @@ endif
 
 ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
 KBUILD_CFLAGS	+= -ffixed-x18
+KBUILD_RUSTFLAGS += -Zfixed-x18
 endif
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)

init/Kconfig

@@ -1909,7 +1909,7 @@ config RUST
 	depends on !MODVERSIONS
 	depends on !GCC_PLUGINS
 	depends on !RANDSTRUCT
-	depends on !SHADOW_CALL_STACK
+	depends on !SHADOW_CALL_STACK || RUSTC_VERSION >= 108000 && UNWIND_PATCH_PAC_INTO_SCS
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
 	help
 	  Enables Rust support in the kernel.