@@ -1810,6 +1810,22 @@ config XEN
help
Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
+config STACKPROTECTOR_PER_TASK
+ bool "Use a unique stack canary value for each task"
+ depends on GCC_PLUGINS && STACKPROTECTOR && SMP
+ select GCC_PLUGIN_ARM_SSP_PER_TASK
+ help
+ Due to the fact that GCC uses an ordinary symbol reference from
+ which to load the value of the stack canary, this value can only
+ change at reboot time on SMP systems, and all tasks running in the
+ kernel's address space are forced to use the same canary value for
+ the entire duration that the system is up.
+
+ Enable this option to switch to a different method that uses a
+ different canary value for each task.
+
+ This is not enabled by default since it relies on a GCC plugin.
+
endmenu
menu "Boot options"
@@ -101,6 +101,7 @@ clean-files += piggy_data lib1funcs.S ashldi3.S bswapsdi2.S \
$(libfdt) $(libfdt_hdrs) hyp-stub.S
KBUILD_CFLAGS += -DDISABLE_BRANCH_PROFILING
+KBUILD_CFLAGS += $(DISABLE_ARM_SSP_PER_TASK_PLUGIN)
ifeq ($(CONFIG_FUNCTION_TRACER),y)
ORIG_CFLAGS := $(KBUILD_CFLAGS)
@@ -6,8 +6,10 @@
* the stack frame and verifying that it hasn't been overwritten when
* returning from the function. The pattern is called stack canary
* and gcc expects it to be defined by a global variable called
- * "__stack_chk_guard" on ARM. This unfortunately means that on SMP
- * we cannot have a different canary value per task.
+ * "__stack_chk_guard" on ARM. This prevents SMP systems from using a
+ * different value for each task unless we enable a GCC plugin that
+ * replaces these symbol references with references to each task's own
+ * value.
*/
#ifndef _ASM_STACKPROTECTOR_H
@@ -16,6 +18,8 @@
#include <linux/random.h>
#include <linux/version.h>
+#include <asm/thread_info.h>
+
extern unsigned long __stack_chk_guard;
/*
@@ -33,7 +37,11 @@ static __always_inline void boot_init_stack_canary(void)
canary ^= LINUX_VERSION_CODE;
current->stack_canary = canary;
+#ifndef CONFIG_STACKPROTECTOR_PER_TASK
__stack_chk_guard = current->stack_canary;
+#else
+ current_thread_info()->stack_canary = current->stack_canary;
+#endif
}
#endif /* _ASM_STACKPROTECTOR_H */
@@ -57,6 +57,9 @@ struct thread_info {
__u32 syscall; /* syscall number */
__u8 used_cp[16]; /* thread used copro */
unsigned long tp_value[2]; /* TLS registers */
+#ifdef CONFIG_STACKPROTECTOR_PER_TASK
+ unsigned long stack_canary;
+#endif
#ifdef CONFIG_CRUNCH
struct crunch_state crunchstate;
#endif
@@ -39,7 +39,7 @@
#include <asm/tls.h>
#include <asm/vdso.h>
-#ifdef CONFIG_STACKPROTECTOR
+#if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
#include <linux/stackprotector.h>
unsigned long __stack_chk_guard __read_mostly;
EXPORT_SYMBOL(__stack_chk_guard);
@@ -267,6 +267,13 @@ copy_thread(unsigned long clone_flags, unsigned long stack_start,
thread_notify(THREAD_NOTIFY_COPY, thread);
+#ifdef CONFIG_STACKPROTECTOR_PER_TASK
+ BUILD_BUG_ON(offsetof(struct thread_info, stack_canary) != 100);
+ BUILD_BUG_ON(THREAD_SIZE != 0x2000);
+
+ thread->stack_canary = p->stack_canary;
+#endif
+
return 0;
}
@@ -36,6 +36,12 @@ ifdef CONFIG_GCC_PLUGIN_STACKLEAK
endif
export DISABLE_STACKLEAK_PLUGIN
+gcc-plugin-$(CONFIG_GCC_PLUGIN_ARM_SSP_PER_TASK) += arm_ssp_per_task_plugin.so
+ifdef CONFIG_GCC_PLUGIN_ARM_SSP_PER_TASK
+ DISABLE_ARM_SSP_PER_TASK_PLUGIN += -fplugin-arg-arm_ssp_per_task_plugin-disable
+endif
+export DISABLE_ARM_SSP_PER_TASK_PLUGIN
+
# 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))
@@ -190,4 +190,8 @@ config STACKLEAK_RUNTIME_DISABLE
runtime to control kernel stack erasing for kernels built with
CONFIG_GCC_PLUGIN_STACKLEAK.
+config GCC_PLUGIN_ARM_SSP_PER_TASK
+ bool
+ depends on GCC_PLUGINS && ARM
+
endif
new file mode 100644
@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "gcc-common.h"
+
+/* TODO: wire these up to the .h files that define them */
+#define THREAD_SIZE 0x2000
+#define TI_STACK_CANARY_OFFSET 100
+
+__visible int plugin_is_GPL_compatible;
+
+static unsigned int arm_pertask_ssp_rtl_execute(void)
+{
+ rtx_insn *insn;
+
+ for (insn = get_insns(); insn; insn = NEXT_INSN(insn)) {
+ const char *sym;
+ rtx body;
+ rtx masked_sp;
+
+ /*
+ * Find a SET insn involving a SYMBOL_REF to __stack_chk_guard
+ */
+ if (!INSN_P(insn))
+ continue;
+ body = PATTERN(insn);
+ if (GET_CODE(body) != SET ||
+ GET_CODE(SET_SRC(body)) != SYMBOL_REF)
+ continue;
+ sym = XSTR(SET_SRC(body), 0);
+ if (strcmp(sym, "__stack_chk_guard"))
+ continue;
+
+ /*
+ * Replace the source of the SET insn with an expression that
+ * produces the address of the copy of the stack canary value
+ * stored in struct thread_info
+ */
+ masked_sp = gen_reg_rtx(Pmode);
+
+ emit_insn_before(gen_rtx_SET(masked_sp,
+ gen_rtx_AND(Pmode,
+ stack_pointer_rtx,
+ GEN_INT(~(THREAD_SIZE - 1)))),
+ insn);
+
+ SET_SRC(body) = gen_rtx_PLUS(Pmode, masked_sp,
+ GEN_INT(TI_STACK_CANARY_OFFSET));
+ }
+ return 0;
+}
+
+#define PASS_NAME arm_pertask_ssp_rtl
+
+#define NO_GATE
+#include "gcc-generate-rtl-pass.h"
+
+__visible int plugin_init(struct plugin_name_args *plugin_info,
+ struct plugin_gcc_version *version)
+{
+ int i;
+
+ if (!plugin_default_version_check(version, &gcc_version)) {
+ error(G_("incompatible gcc/plugin versions"));
+ return 1;
+ }
+
+ for (i = 0; i < plugin_info->argc; i++) {
+ if (strcmp(plugin_info->argv[i].key, "disable") == 0)
+ return 0;
+ }
+
+ PASS_INFO(arm_pertask_ssp_rtl, "expand", 1, PASS_POS_INSERT_AFTER);
+
+ register_callback(plugin_info->base_name, PLUGIN_PASS_MANAGER_SETUP,
+ NULL, &arm_pertask_ssp_rtl_pass_info);
+
+ return 0;
+}
On ARM, we currently only change the value of the stack canary when switching tasks if the kernel was built for UP. On SMP kernels, this is impossible since the stack canary value is obtained via a global symbol reference, which means a) all running tasks on all CPUs must use the same value b) we can only modify the value when no kernel stack frames are live, which is effectively never. So instead, use a GCC plugin to add a RTL pass that replaces each reference to the address of the __stack_chk_guard symbol with an expression that produces the address of the 'stack_canary' field that is added to struct thread_info. This way, each task will use its own randomized value. Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> --- Known issues: - The plugin requires the values of THREAD_SIZE and the offset of stack_canary in struct thread_info at a really early stage in the build, so I had to redefine them in the plugin source. Questions: - Are there any execution contexts except for the decompressor and the EFI stub where we should disable this? KVM perhaps? Comments and test results welcome. arch/arm/Kconfig | 16 ++++ arch/arm/boot/compressed/Makefile | 1 + arch/arm/include/asm/stackprotector.h | 12 ++- arch/arm/include/asm/thread_info.h | 3 + arch/arm/kernel/process.c | 9 ++- scripts/Makefile.gcc-plugins | 6 ++ scripts/gcc-plugins/Kconfig | 4 + scripts/gcc-plugins/arm_ssp_per_task_plugin.c | 78 ++++++++++++++++++++ 8 files changed, 126 insertions(+), 3 deletions(-)