[PATCH] [RFC/RFT,V2] ARM64: SCS: Add gcc plugin to support Shadow Call Stack

[Dan Li]

This patch supports gcc-based SCS protection on ARM64 by adding a plugin.

For each function that x30 will be pushed onto the stack during execution,
this plugin, similar to gcc's pac implementation, will normally:
1) insert "str x30, [x18], #8" at the beginning of function's prologue
2) insert "ldr x30, [x18, #-8]!" immediately before function's
epilogue return/sibling calls

If pac is enabled, scs push/pop will be inserted between paciasp/autiasp.

At present, this patch has been successfully compiled(based on defconfig
with kernel 5.14) in the following gcc versions(if plugin is supported)
and startup normally with commands:

make ARCH=arm64 defconfig
./scripts/config -e CONFIG_GCC_PLUGINS -e CONFIG_SHADOW_CALL_STACK \
-e CONFIG_GCC_PLUGIN_SHADOW_CALL_STACK
make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu-

Tested gcc version:
* 6.3.1
* 7.3.1
* 7.5.0
* 8.2.1
* 9.2.0
* 10.3.1

Signed-off-by: Dan Li <ashimida@xxxxxxxxxxxxxxxxx>

---
V2:
- fix incorrect config dependency
- add support for DYNAMIC_FTRACE_WITH_REGS
- add support for ARM64_PTR_AUTH_KERNEL/ARM64_BTI_KERNEL

FYI:
This function can be used to test whether the shadow stack is effective:

//noinline void __noscs scs_test(void)
noinline void scs_test(void)
{
    register unsigned long *sp asm("sp");
    unsigned long * lr = sp + 1;

    asm volatile("":::"x30");
    *lr = 0;
}

when compiled with:
CONFIG_DYNAMIC_FTRACE_WITH_REGS=y
CONFIG_ARM64_PTR_AUTH_KERNEL=y
CONFIG_ARM64_BTI_KERNEL=y

ffff800010013b60 <scs_test>:
ffff800010013b60:       d503245f        bti     c
ffff800010013b64:       d503201f        nop
ffff800010013b68:       d503201f        nop
ffff800010013b6c:       d503233f        paciasp
ffff800010013b70:       f800865e        str     x30, [x18], #8
ffff800010013b74:       a9bf7bfd        stp     x29, x30, [sp, #-16]!
ffff800010013b78:       910003fd        mov     x29, sp
ffff800010013b7c:       910003e0        mov     x0, sp
ffff800010013b80:       f900041f        str     xzr, [x0, #8]
ffff800010013b84:       a8c17bfd        ldp     x29, x30, [sp], #16
ffff800010013b88:       f85f8e5e        ldr     x30, [x18, #-8]!
ffff800010013b8c:       d50323bf        autiasp
ffff800010013b90:       d65f03c0        ret

If SCS protection is enabled, this function will return normally.
If the function has __noscs attribute (scs disabled), it will crash due to 0
address access.
---
 Makefile                               |  20 ++-
 arch/Kconfig                           |  12 +-
 arch/arm64/Kconfig                     |   4 +-
 include/linux/compiler-gcc.h           |   4 +
 scripts/Makefile.gcc-plugins           |   7 +
 scripts/gcc-plugins/Kconfig            |   9 +
 scripts/gcc-plugins/arm64_scs_plugin.c | 299 +++++++++++++++++++++++++++++++++
 scripts/gcc-plugins/gcc-common.h       |   4 +
 8 files changed, 345 insertions(+), 14 deletions(-)
 create mode 100644 scripts/gcc-plugins/arm64_scs_plugin.c

diff --git a/Makefile b/Makefile
index 61741e9..8039e61 100644
--- a/Makefile
+++ b/Makefile
@@ -923,12 +923,6 @@ KBUILD_CFLAGS_KERNEL += -ffunction-sections -fdata-sections
 LDFLAGS_vmlinux += --gc-sections
 endif
 
-ifdef CONFIG_SHADOW_CALL_STACK
-CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
-KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
-export CC_FLAGS_SCS
-endif
-
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
@@ -1034,6 +1028,20 @@ include-$(CONFIG_GCC_PLUGINS)	+= scripts/Makefile.gcc-plugins
 
 include $(addprefix $(srctree)/, $(include-y))
 
+ifdef CONFIG_SHADOW_CALL_STACK
+
+ifdef CONFIG_CC_IS_CLANG
+CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
+endif
+
+ifdef CONFIG_CC_IS_GCC
+CC_FLAGS_SCS	:= $(ENABLE_SHADOW_CALL_STACK_PLUGIN)
+endif
+
+KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
+export CC_FLAGS_SCS
+endif
+
 # scripts/Makefile.gcc-plugins is intentionally included last.
 # Do not add $(call cc-option,...) below this line. When you build the kernel
 # from the clean source tree, the GCC plugins do not exist at this point.
diff --git a/arch/Kconfig b/arch/Kconfig
index 98db634..de61dbc 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -593,14 +593,14 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
 	  switching.
 
 config SHADOW_CALL_STACK
-	bool "Clang Shadow Call Stack"
-	depends on CC_IS_CLANG && ARCH_SUPPORTS_SHADOW_CALL_STACK
+	bool "Shadow Call Stack"
+	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
 	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
 	help
-	  This option enables Clang's Shadow Call Stack, which uses a
-	  shadow stack to protect function return addresses from being
-	  overwritten by an attacker. More information can be found in
-	  Clang's documentation:
+	  This option enables Clang/GCC plugin's Shadow Call Stack, which
+	  uses a shadow stack to protect function return addresses from
+	  being overwritten by an attacker. More information can be found
+	  in Clang's documentation:
 
 	    https://clang.llvm.org/docs/ShadowCallStack.html
 
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 62c3c1d..da2da8c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -81,7 +81,7 @@ config ARM64
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC
 	select ARCH_SUPPORTS_HUGETLBFS
 	select ARCH_SUPPORTS_MEMORY_FAILURE
-	select ARCH_SUPPORTS_SHADOW_CALL_STACK if CC_HAVE_SHADOW_CALL_STACK
+	select ARCH_SUPPORTS_SHADOW_CALL_STACK if (CC_HAVE_SHADOW_CALL_STACK || GCC_PLUGIN_SHADOW_CALL_STACK)
 	select ARCH_SUPPORTS_LTO_CLANG if CPU_LITTLE_ENDIAN
 	select ARCH_SUPPORTS_LTO_CLANG_THIN
 	select ARCH_SUPPORTS_CFI_CLANG
@@ -1062,7 +1062,7 @@ config ARCH_HAS_FILTER_PGPROT
 
 # Supported by clang >= 7.0
 config CC_HAVE_SHADOW_CALL_STACK
-	def_bool $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18)
+	def_bool (CC_IS_CLANG && $(cc-option, -fsanitize=shadow-call-stack -ffixed-x18))
 
 config PARAVIRT
 	bool "Enable paravirtualization code"
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index cb9217f..426c8e5 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -50,6 +50,10 @@
 #define __latent_entropy __attribute__((latent_entropy))
 #endif
 
+#if defined(SHADOW_CALL_STACK_PLUGIN) && !defined(__CHECKER__)
+#define __noscs __attribute__((no_shadow_call_stack))
+#endif
+
 /*
  * calling noreturn functions, __builtin_unreachable() and __builtin_trap()
  * confuse the stack allocation in gcc, leading to overly large stack
diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index 952e468..b45bd8c 100644
--- a/scripts/Makefile.gcc-plugins
+++ b/scripts/Makefile.gcc-plugins
@@ -46,6 +46,13 @@ ifdef CONFIG_GCC_PLUGIN_ARM_SSP_PER_TASK
 endif
 export DISABLE_ARM_SSP_PER_TASK_PLUGIN
 
+gcc-plugin-$(CONFIG_GCC_PLUGIN_SHADOW_CALL_STACK) += arm64_scs_plugin.so
+gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_SHADOW_CALL_STACK)	\
+		+= -DSHADOW_CALL_STACK_PLUGIN
+ifdef CONFIG_GCC_PLUGIN_SHADOW_CALL_STACK
+    ENABLE_SHADOW_CALL_STACK_PLUGIN += -fplugin-arg-arm64_scs_plugin-enable
+endif
+
 # All the plugin CFLAGS are collected here in case a build target needs to
 # filter them out of the KBUILD_CFLAGS.
 GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
diff --git a/scripts/gcc-plugins/Kconfig b/scripts/gcc-plugins/Kconfig
index ab9eb4c..92f7f76 100644
--- a/scripts/gcc-plugins/Kconfig
+++ b/scripts/gcc-plugins/Kconfig
@@ -19,6 +19,15 @@ menuconfig GCC_PLUGINS
 
 if GCC_PLUGINS
 
+config GCC_PLUGIN_SHADOW_CALL_STACK
+	bool "Plugin for ARM64 Shadow Call Stack"
+	depends on (!CC_HAVE_SHADOW_CALL_STACK) && ARM64
+	help
+	  This plugin is used to support kernel CONFIG_SHADOW_CALL_STACK
+	  compiled by gcc. Its principle is basically the same as that of
+	  CLANG.
+	  For more information, please refer to "config SHADOW_CALL_STACK"
+
 config GCC_PLUGIN_CYC_COMPLEXITY
 	bool "Compute the cyclomatic complexity of a function" if EXPERT
 	depends on !COMPILE_TEST	# too noisy
diff --git a/scripts/gcc-plugins/arm64_scs_plugin.c b/scripts/gcc-plugins/arm64_scs_plugin.c
new file mode 100644
index 0000000..bb72baa
--- /dev/null
+++ b/scripts/gcc-plugins/arm64_scs_plugin.c
@@ -0,0 +1,299 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "gcc-common.h"
+
+#define v_info(fmt, ...)							\
+	do {									\
+		if (verbose)							\
+			fprintf(stderr, "[SCS]:" fmt,  ## __VA_ARGS__);	\
+	} while (0)
+
+#define NOSCS_ATTR_STR  "no_shadow_call_stack"
+#define SCS_ASM_PUSH_STR "str x30, [x18], #8\n\t"
+#define SCS_ASM_POP_STR  "ldr x30, [x18, #-8]!\n\t"
+
+__visible int plugin_is_GPL_compatible;
+
+static struct plugin_info arm64_scs_plugin_info = {
+	.version	= "20210926vanilla",
+	.help		= "enable\tactivate plugin\n"
+			  "verbose\tprint all debug infos\n",
+};
+
+static bool verbose;
+
+#if BUILDING_GCC_VERSION >= 10001
+enum insn_code paciasp_num = CODE_FOR_paciasp;
+enum insn_code autiasp_num = CODE_FOR_autiasp;
+#elif BUILDING_GCC_VERSION >= 7003
+enum insn_code paciasp_num = CODE_FOR_pacisp;
+enum insn_code autiasp_num = CODE_FOR_autisp;
+#else
+enum insn_code paciasp_num = CODE_FOR_nothing;
+enum insn_code autiasp_num = CODE_FOR_nothing;
+#define TARGET_ARMV8_3 0
+#endif
+
+static rtx_insn * (*old_gen_prologue)(void);
+static rtx_insn * (*old_gen_epilogue)(void);
+static rtx_insn * (*old_gen_sibcall_epilogue)(void);
+
+static rtx gen_scs_push(location_t loc)
+{
+	rtx insn = gen_rtx_ASM_INPUT_loc(VOIDmode, ggc_strdup(SCS_ASM_PUSH_STR), loc);
+
+	MEM_VOLATILE_P(insn) = 1;
+	return insn;
+}
+
+static rtx gen_scs_pop(location_t loc)
+{
+	rtx insn = gen_rtx_ASM_INPUT_loc(VOIDmode, ggc_strdup(SCS_ASM_POP_STR), loc);
+
+	MEM_VOLATILE_P(insn) = 1;
+	return insn;
+}
+
+static bool scs_func_ignored(void)
+{
+	bool is_ignored;
+
+#if BUILDING_GCC_VERSION >= 8002
+	is_ignored = !cfun->machine->frame.emit_frame_chain;
+#else
+	is_ignored = !frame_pointer_needed;
+#endif
+
+	/*
+	 * Functions that do not push LR into stack are not protected.
+	 * Functions that call __builin_eh_return is not protected(consistent with gcc's PAC).
+	 */
+	if (is_ignored || crtl->calls_eh_return) {
+		v_info("No protection code inserted into func:%s in file:%s\n",
+			get_name(current_function_decl), main_input_filename);
+		return 1;
+	}
+
+	/* Functions with attribute NOSCS_ATTR_STR need to be unprotected */
+	if (lookup_attribute(NOSCS_ATTR_STR, DECL_ATTRIBUTES(current_function_decl))) {
+		v_info("No protection code inserted into %s func:%s in file:%s\n", NOSCS_ATTR_STR,
+				get_name(current_function_decl), main_input_filename);
+		return 1;
+	}
+
+	return 0;
+}
+
+static rtx_insn *search_insn(enum insn_code code, rtx_insn *seq)
+{
+	rtx_insn *insn;
+
+	for (insn = get_insns(); insn; insn = NEXT_INSN(insn)) {
+		if (code == recog(PATTERN(insn), insn, 0))
+			return insn;
+	}
+
+	return NULL;
+}
+
+static bool scs_return_address_signing_enabled(void)
+{
+#if BUILDING_GCC_VERSION >= 7003
+	return aarch64_return_address_signing_enabled();
+#else
+	return false;
+#endif
+}
+
+static rtx_insn *scs_gen_prologue(void)
+{
+	rtx_insn *seq = NULL, *mark;
+	rtx tmp;
+	bool ret_sign_enabled;
+
+	if (old_gen_prologue)
+		seq = old_gen_prologue();
+
+	if ((!seq) || scs_func_ignored())
+		return seq;
+
+	ret_sign_enabled = scs_return_address_signing_enabled();
+	tmp = gen_scs_push(RESERVED_LOCATION_COUNT);
+
+	start_sequence();
+	emit_insn(seq);
+
+	if (ret_sign_enabled) {
+		/* For functions with pac enabled, insert scs push after the 'paciasp' insn */
+		mark = search_insn(paciasp_num, get_insns());
+		if (!mark)
+			error(G_("Non-standard insn seqs found:\n"
+				"__noscs attr should be added on func:%s,file:%s\n"),
+				get_name(current_function_decl), main_input_filename);
+
+		emit_insn_after(tmp, mark);
+	} else {
+		/* For functions that do not enable pac, insert scs push at the start of insns */
+		mark = get_insns();
+		emit_insn_before(tmp, mark);
+	}
+
+	seq = get_insns();
+	end_sequence();
+	return seq;
+}
+
+static rtx_insn *scs_gen_epilogue(void)
+{
+	rtx_insn *seq = NULL, *mark;
+	rtx tmp;
+	bool ret_sign_enabled;
+
+	if (old_gen_epilogue)
+		seq = old_gen_epilogue();
+
+	if ((!seq) || scs_func_ignored())
+		return seq;
+
+	ret_sign_enabled = scs_return_address_signing_enabled();
+	tmp = gen_scs_pop(RESERVED_LOCATION_COUNT);
+
+	start_sequence();
+	emit_insn(seq);
+
+	if (ret_sign_enabled && (!TARGET_ARMV8_3)) {
+		/* For functions with pac enabled, if 'autiasp' is used in epilogue
+		 * (!TARGET_ARMV8_3), scs pop should inserted before this insn.
+		 */
+		mark = search_insn(autiasp_num, get_insns());
+	} else {
+		/* For functions do not enabled pac or used 'retaa' as pac check,
+		 * scs pop inserted before the last 'return" insn
+		 */
+		mark = get_last_insn();
+	}
+
+	if (!mark)
+		error(G_("Non-standard insn seqs found:\n"
+			"__noscs attr should be added on func:%s,file:%s\n"),
+			get_name(current_function_decl), main_input_filename);
+
+	emit_insn_before(tmp, mark);
+
+	seq = get_insns();
+	end_sequence();
+	return seq;
+}
+
+static rtx_insn *scs_gen_sibcall_epilogue(void)
+{
+	rtx_insn *seq = NULL, *mark;
+	rtx tmp;
+	bool ret_sign_enabled;
+
+	if (old_gen_sibcall_epilogue)
+		seq = old_gen_sibcall_epilogue();
+
+	if ((!seq) || scs_func_ignored())
+		return seq;
+
+	ret_sign_enabled = scs_return_address_signing_enabled();
+	tmp = gen_scs_pop(RESERVED_LOCATION_COUNT);
+
+	start_sequence();
+	emit_insn(seq);
+
+	if (ret_sign_enabled) {
+		/* If pac is enabled, sibling_call will always use 'autiasp' as pac check */
+		mark = search_insn(autiasp_num, get_insns());
+		if (!mark)
+			error(G_("Non-standard insn seqs found:\n"
+				"__noscs attr should be added on func:%s,file:%s\n"),
+				get_name(current_function_decl), main_input_filename);
+		emit_insn_before(tmp, mark);
+	} else {
+		/* If pac is disabled, insert scs pop at the end of insns */
+		mark = get_last_insn();
+		emit_insn_after(tmp, mark);
+	}
+
+	seq = get_insns();
+	end_sequence();
+
+	return seq;
+}
+
+static void callback_before_start_unit(void *gcc_data __unused, void *user_data __unused)
+{
+	old_gen_prologue = targetm.gen_prologue;
+	old_gen_epilogue = targetm.gen_epilogue;
+	old_gen_sibcall_epilogue = targetm.gen_sibcall_epilogue;
+
+	targetm.gen_prologue = scs_gen_prologue;
+	targetm.gen_epilogue = scs_gen_epilogue;
+	targetm.gen_sibcall_epilogue = scs_gen_sibcall_epilogue;
+}
+
+static tree handle_noscs_attribute(tree *node, tree name, tree args __unused, int flags,
+		bool *no_add_attrs)
+{
+	/* NOSCS_ATTR_STR can only be used for function declarations */
+	switch (TREE_CODE(*node)) {
+	case FUNCTION_DECL:
+		break;
+	default:
+		error(G_("%qE attribute can be applies to function decl only (%qE)"), name, *node);
+		gcc_unreachable();
+	}
+
+	*no_add_attrs = false;
+	return NULL_TREE;
+}
+
+static struct attribute_spec noscs_attr = {};
+
+static void scs_register_attributes(void *event_data __unused, void *data __unused)
+{
+	noscs_attr.name	= NOSCS_ATTR_STR;
+	noscs_attr.decl_required = true;
+	noscs_attr.handler = handle_noscs_attribute;
+	register_attribute(&noscs_attr);
+}
+
+__visible int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version)
+{
+	int i;
+	bool enable = false;
+	const char * const plugin_name = plugin_info->base_name;
+	const int argc = plugin_info->argc;
+	const struct plugin_argument * const argv = plugin_info->argv;
+
+	if (!plugin_default_version_check(version, &gcc_version)) {
+		error(G_("Incompatible gcc/plugin versions"));
+		return 1;
+	}
+
+	for (i = 0; i < argc; ++i) {
+		if (!strcmp(argv[i].key, "enable")) {
+			enable = true;
+			continue;
+		}
+		if (!strcmp(argv[i].key, "verbose")) {
+			verbose = true;
+			continue;
+		}
+		error(G_("unknown option '-fplugin-arg-%s-%s'"), plugin_name, argv[i].key);
+	}
+
+	if (!enable) {
+		v_info("Plugin disabled for file:%s\n", main_input_filename);
+		return 0;
+	}
+
+	register_callback(plugin_name, PLUGIN_INFO, NULL, &arm64_scs_plugin_info);
+
+	register_callback(plugin_name, PLUGIN_ATTRIBUTES, scs_register_attributes, NULL);
+
+	register_callback(plugin_name, PLUGIN_START_UNIT, callback_before_start_unit, NULL);
+
+	return 0;
+}
diff --git a/scripts/gcc-plugins/gcc-common.h b/scripts/gcc-plugins/gcc-common.h
index 0c08761..7251b00 100644
--- a/scripts/gcc-plugins/gcc-common.h
+++ b/scripts/gcc-plugins/gcc-common.h
@@ -27,6 +27,7 @@
 #include "except.h"
 #include "function.h"
 #include "toplev.h"
+#include "insn-codes.h"
 #if BUILDING_GCC_VERSION >= 5000
 #include "expr.h"
 #endif
@@ -535,6 +536,9 @@ static inline void ipa_remove_stmt_references(symtab_node *referring_node, gimpl
 }
 #endif
 
+/* RTL related */
+extern int recog(rtx, rtx_insn *, int *);
+
 #if BUILDING_GCC_VERSION < 6000
 #define get_inner_reference(exp, pbitsize, pbitpos, poffset, pmode, punsignedp, preversep, pvolatilep, keep_aligning)	\
 	get_inner_reference(exp, pbitsize, pbitpos, poffset, pmode, punsignedp, pvolatilep, keep_aligning)
-- 
2.7.4