[PATCH v2] rust: add flags for shadow call stack sanitizer

Alice Ryhl
2024-03-05T11:58:54Z

Add flags to support the shadow call stack sanitizer, both in the
dynamic and non-dynamic modes.

Right now, the compiler will emit the warning "unknown feature specified
for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
passes it to the codegen backend, so the flag will work just fine. Once
rustc starts recognizing the flag (or provides another way to enable the
feature), it will stop emitting this warning. See [1] for the relevant
issue.

Currently, the compiler thinks that the aarch64-unknown-none target
doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
you enable shadow call stack in non-dynamic mode. However, I still think
it is reasonable to add the flag now, as it will at least fail the build
when using an invalid configuration, until the Rust compiler is fixed to
list -Zsanitizer=shadow-call-stack as supported for the target. See [2]
for the feature request to add this.

I have tested this change with Rust Binder on an Android device using
CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=+reserve-x18 flag, the
phone crashes immediately on boot, and with the flag, the phone appears
to work normally.

This contains a TODO to add the -Zuse-sync-unwind=n flag. The flag
defaults to n, so it isn't a problem today, but the flag is unstable, so
the default could change in a future compiler release.

Link: https://github.com/rust-lang/rust/issues/121970 [1]
Link: https://github.com/rust-lang/rust/issues/121972 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
This patch raises the question of whether we should change the Rust
aarch64 support to use a custom target.json specification. If we do
that, then we can fix both the warning for dynamic SCS and the
build-failure for non-dynamic SCS without waiting for a new version of
rustc with the mentioned issues fixed.
---
Changes in v2:
- Add -Cforce-unwind-tables flag.
- Link to v1: https://lore.kernel.org/r/20240304-shadow-call-stack-v1-1-f055eaf40a2c@google.com
---
Makefile | 1 +
arch/arm64/Makefile | 4 ++++
2 files changed, 5 insertions(+)

diff --git a/Makefile b/Makefile
index 0e36eff14608..345066643a76 100644
--- a/Makefile
+++ b/Makefile
@@ -936,6 +936,7 @@ 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
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index a88cdf910687..9bd5522c18e9 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -48,9 +48,12 @@ 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
+# TODO: Pass -Zuse-sync-unwind=n once we upgrade to Rust 1.77.0
+KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y
endif

ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
@@ -103,6 +106,7 @@ endif

ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
KBUILD_CFLAGS += -ffixed-x18
+KBUILD_RUSTFLAGS += -Ctarget-feature=+reserve-x18
endif

ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)

---
base-commit: 90d35da658da8cff0d4ecbb5113f5fac9d00eb72
change-id: 20240304-shadow-call-stack-9c197a4361d9

Best regards,
--
Alice Ryhl <aliceryhl@google.com>