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 <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Fixed indentation using spaces. - Miguel ]
Signed-off-by: Miguel Ojeda <[email protected]>

Makefile

@@ -929,6 +929,7 @@ 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/Kconfig

@@ -231,7 +231,7 @@ config ARM64
 	select HAVE_FUNCTION_ARG_ACCESS_API
 	select MMU_GATHER_RCU_TABLE_FREE
 	select HAVE_RSEQ
-	select HAVE_RUST if CPU_LITTLE_ENDIAN
+	select HAVE_RUST if RUSTC_SUPPORTS_ARM64
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_KPROBES
@@ -265,6 +265,18 @@ config ARM64
 	help
 	  ARM 64-bit (AArch64) Linux support.
 
+config RUSTC_SUPPORTS_ARM64
+	def_bool y
+	depends on CPU_LITTLE_ENDIAN
+	# Shadow call stack is only supported on certain rustc versions.
+	#
+	# When using the UNWIND_PATCH_PAC_INTO_SCS option, rustc version 1.80+ is
+	# required due to use of the -Zfixed-x18 flag.
+	#
+	# Otherwise, rustc version 1.82+ is required due to use of the
+	# -Zsanitizer=shadow-call-stack flag.
+	depends on !SHADOW_CALL_STACK || RUSTC_VERSION >= 108200 || RUSTC_VERSION >= 108000 && UNWIND_PATCH_PAC_INTO_SCS
+
 config CLANG_SUPPORTS_DYNAMIC_FTRACE_WITH_ARGS
 	def_bool CC_IS_CLANG
 	# https://github.com/ClangBuiltLinux/linux/issues/1507

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)

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

init/Kconfig

@@ -1909,7 +1909,6 @@ config RUST
 	depends on !MODVERSIONS
 	depends on !GCC_PLUGIN_RANDSTRUCT
 	depends on !RANDSTRUCT
-	depends on !SHADOW_CALL_STACK
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
 	help
 	  Enables Rust support in the kernel.