Subject: [PATCH v2 05/17] add support for Clang's Shadow Call Stack (SCS)

From: [email-masked]
Date: Thu, 24 Oct 2019 15:51:20 -0700

This change adds generic support for Clang's Shadow Call Stack,
which uses a shadow stack to protect return addresses from being
overwritten by an attacker. Details are available here:

  https://clang.llvm.org/docs/ShadowCallStack.html

Note that security guarantees in the kernel differ from the
ones documented for user space. The kernel must store addresses
of shadow stacks used by other tasks and interrupt handlers in
memory, which means an attacker capable reading and writing
arbitrary memory may be able to locate them and hijack control
flow by modifying shadow stacks that are not currently in use.

Signed-off-by: Sami Tolvanen <[email-masked]>
---
 Makefile                       |   6 ++
 arch/Kconfig                   |  33 +++++++
 include/linux/compiler-clang.h |   6 ++
 include/linux/compiler_types.h |   4 +
 include/linux/scs.h            |  78 +++++++++++++++++
 init/init_task.c               |   8 ++
 kernel/Makefile                |   1 +
 kernel/fork.c                  |   9 ++
 kernel/sched/core.c            |   2 +
 kernel/sched/sched.h           |   1 +
 kernel/scs.c                   | 155 +++++++++++++++++++++++++++++++++
 11 files changed, 303 insertions(+)
 create mode 100644 include/linux/scs.h
 create mode 100644 kernel/scs.c

diff --git a/Makefile b/Makefile
index 5475cdb6d57d..2b5c59fb18f2 100644
--- a/Makefile
+++ b/Makefile
@@ -846,6 +846,12 @@ ifdef CONFIG_LIVEPATCH
 KBUILD_CFLAGS += $(call cc-option, -flive-patching=inline-clone)
 endif
 
+ifdef CONFIG_SHADOW_CALL_STACK
+CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
+KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
+export CC_FLAGS_SCS
+endif
+
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
 
diff --git a/arch/Kconfig b/arch/Kconfig
index 5f8a5d84dbbe..5e34cbcd8d6a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -521,6 +521,39 @@ config STACKPROTECTOR_STRONG
 	  about 20% of all kernel functions, which increases the kernel code
 	  size by about 2%.
 
+config ARCH_SUPPORTS_SHADOW_CALL_STACK
+	bool
+	help
+	  An architecture should select this if it supports Clang's Shadow
+	  Call Stack, has asm/scs.h, and implements runtime support for shadow
+	  stack switching.
+
+config SHADOW_CALL_STACK_VMAP
+	bool
+	depends on SHADOW_CALL_STACK
+	help
+	  Use virtually mapped shadow call stacks. Selecting this option
+	  provides better stack exhaustion protection, but increases per-thread
+	  memory consumption as a full page is allocated for each shadow stack.
+
+config SHADOW_CALL_STACK
+	bool "Clang Shadow Call Stack"
+	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
+	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 from
+	  Clang's documentation:
+
+	    https://clang.llvm.org/docs/ShadowCallStack.html
+
+	  Note that security guarantees in the kernel differ from the ones
+	  documented for user space. The kernel must store addresses of shadow
+	  stacks used by other tasks and interrupt handlers in memory, which
+	  means an attacker capable reading and writing arbitrary memory may
+	  be able to locate them and hijack control flow by modifying shadow
+	  stacks that are not currently in use.
+
 config HAVE_ARCH_WITHIN_STACK_FRAMES
 	bool
 	help
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 333a6695a918..afe5e24088b2 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -42,3 +42,9 @@
  * compilers, like ICC.
  */
 #define barrier() __asm__ __volatile__("" : : : "memory")
+
+#if __has_feature(shadow_call_stack)
+# define __noscs	__attribute__((no_sanitize("shadow-call-stack")))
+#else
+# define __noscs
+#endif
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 72393a8c1a6c..be5d5be4b1ae 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -202,6 +202,10 @@ struct ftrace_likely_data {
 # define randomized_struct_fields_end
 #endif
 
+#ifndef __noscs
+# define __noscs
+#endif
+
 #ifndef asm_volatile_goto
 #define asm_volatile_goto(x...) asm goto(x)
 #endif
diff --git a/include/linux/scs.h b/include/linux/scs.h
new file mode 100644
index 000000000000..c8b0ccfdd803
--- /dev/null
+++ b/include/linux/scs.h
@@ -0,0 +1,78 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Shadow Call Stack support.
+ *
+ * Copyright (C) 2018 Google LLC
+ */
+
+#ifndef _LINUX_SCS_H
+#define _LINUX_SCS_H
+
+#include <linux/gfp.h>
+#include <linux/sched.h>
+#include <asm/page.h>
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+
+#define SCS_SIZE	1024
+#define SCS_END_MAGIC	0xaf0194819b1635f6UL
+
+#define GFP_SCS		(GFP_KERNEL | __GFP_ZERO)
+
+static inline void *task_scs(struct task_struct *tsk)
+{
+	return task_thread_info(tsk)->shadow_call_stack;
+}
+
+static inline void task_set_scs(struct task_struct *tsk, void *s)
+{
+	task_thread_info(tsk)->shadow_call_stack = s;
+}
+
+extern void scs_init(void);
+extern void scs_task_init(struct task_struct *tsk);
+extern void scs_task_reset(struct task_struct *tsk);
+extern int scs_prepare(struct task_struct *tsk, int node);
+extern bool scs_corrupted(struct task_struct *tsk);
+extern void scs_release(struct task_struct *tsk);
+
+#else /* CONFIG_SHADOW_CALL_STACK */
+
+static inline void *task_scs(struct task_struct *tsk)
+{
+	return 0;
+}
+
+static inline void task_set_scs(struct task_struct *tsk, void *s)
+{
+}
+
+static inline void scs_init(void)
+{
+}
+
+static inline void scs_task_init(struct task_struct *tsk)
+{
+}
+
+static inline void scs_task_reset(struct task_struct *tsk)
+{
+}
+
+static inline int scs_prepare(struct task_struct *tsk, int node)
+{
+	return 0;
+}
+
+static inline bool scs_corrupted(struct task_struct *tsk)
+{
+	return false;
+}
+
+static inline void scs_release(struct task_struct *tsk)
+{
+}
+
+#endif /* CONFIG_SHADOW_CALL_STACK */
+
+#endif /* _LINUX_SCS_H */
diff --git a/init/init_task.c b/init/init_task.c
index 9e5cbe5eab7b..cbd40460e903 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -11,6 +11,7 @@
 #include <linux/mm.h>
 #include <linux/audit.h>
 #include <linux/numa.h>
+#include <linux/scs.h>
 
 #include <asm/pgtable.h>
 #include <linux/uaccess.h>
@@ -184,6 +185,13 @@ struct task_struct init_task
 };
 EXPORT_SYMBOL(init_task);
 
+#ifdef CONFIG_SHADOW_CALL_STACK
+unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)] __init_task_data
+		__aligned(SCS_SIZE) = {
+	[(SCS_SIZE / sizeof(long)) - 1] = SCS_END_MAGIC
+};
+#endif
+
 /*
  * Initial thread structure. Alignment of this is handled by a special
  * linker map entry.
diff --git a/kernel/Makefile b/kernel/Makefile
index daad787fb795..313dbd44d576 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -102,6 +102,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
 obj-$(CONFIG_IRQ_WORK) += irq_work.o
 obj-$(CONFIG_CPU_PM) += cpu_pm.o
 obj-$(CONFIG_BPF) += bpf/
+obj-$(CONFIG_SHADOW_CALL_STACK) += scs.o
 
 obj-$(CONFIG_PERF_EVENTS) += events/
 
diff --git a/kernel/fork.c b/kernel/fork.c
index bcdf53125210..ae7ebe9f0586 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -94,6 +94,7 @@
 #include <linux/livepatch.h>
 #include <linux/thread_info.h>
 #include <linux/stackleak.h>
+#include <linux/scs.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -451,6 +452,8 @@ void put_task_stack(struct task_struct *tsk)
 
 void free_task(struct task_struct *tsk)
 {
+	scs_release(tsk);
+
 #ifndef CONFIG_THREAD_INFO_IN_TASK
 	/*
 	 * The task is finally done with both the stack and thread_info,
@@ -834,6 +837,8 @@ void __init fork_init(void)
 			  NULL, free_vm_stack_cache);
 #endif
 
+	scs_init();
+
 	lockdep_init_task(&init_task);
 	uprobes_init();
 }
@@ -907,6 +912,7 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
 	clear_user_return_notifier(tsk);
 	clear_tsk_need_resched(tsk);
 	set_task_stack_end_magic(tsk);
+	scs_task_init(tsk);
 
 #ifdef CONFIG_STACKPROTECTOR
 	tsk->stack_canary = get_random_canary();
@@ -2022,6 +2028,9 @@ static __latent_entropy struct task_struct *copy_process(
 				 args->tls);
 	if (retval)
 		goto bad_fork_cleanup_io;
+	retval = scs_prepare(p, node);
+	if (retval)
+		goto bad_fork_cleanup_thread;
 
 	stackleak_task_init(p);
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index dd05a378631a..e7faeb383008 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6013,6 +6013,8 @@ void init_idle(struct task_struct *idle, int cpu)
 	raw_spin_lock_irqsave(&idle->pi_lock, flags);
 	raw_spin_lock(&rq->lock);
 
+	scs_task_reset(idle);
+
 	__sched_fork(0, idle);
 	idle->state = TASK_RUNNING;
 	idle->se.exec_start = sched_clock();
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 0db2c1b3361e..c153003a011c 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -58,6 +58,7 @@
 #include <linux/profile.h>
 #include <linux/psi.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/scs.h>
 #include <linux/security.h>
 #include <linux/stop_machine.h>
 #include <linux/suspend.h>
diff --git a/kernel/scs.c b/kernel/scs.c
new file mode 100644
index 000000000000..383d29e8c199
--- /dev/null
+++ b/kernel/scs.c
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Shadow Call Stack support.
+ *
+ * Copyright (C) 2019 Google LLC
+ */
+
+#include <linux/cpuhotplug.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <linux/scs.h>
+#include <linux/vmalloc.h>
+#include <asm/scs.h>
+
+static inline void *__scs_base(struct task_struct *tsk)
+{
+	return (void *)((uintptr_t)task_scs(tsk) & ~(SCS_SIZE - 1));
+}
+
+#ifdef CONFIG_SHADOW_CALL_STACK_VMAP
+
+/* Keep a cache of shadow stacks */
+#define SCS_CACHE_SIZE 2
+static DEFINE_PER_CPU(void *, scs_cache[SCS_CACHE_SIZE]);
+
+static void *scs_alloc(int node)
+{
+	int i;
+
+	for (i = 0; i < SCS_CACHE_SIZE; i++) {
+		void *s;
+
+		s = this_cpu_xchg(scs_cache[i], NULL);
+		if (s) {
+			memset(s, 0, SCS_SIZE);
+			return s;
+		}
+	}
+
+	BUILD_BUG_ON(SCS_SIZE > PAGE_SIZE);
+
+	return __vmalloc_node_range(PAGE_SIZE, SCS_SIZE,
+				    VMALLOC_START, VMALLOC_END,
+				    GFP_SCS, PAGE_KERNEL, 0,
+				    node, __builtin_return_address(0));
+}
+
+static void scs_free(void *s)
+{
+	int i;
+
+	for (i = 0; i < SCS_CACHE_SIZE; i++) {
+		if (this_cpu_cmpxchg(scs_cache[i], 0, s) != 0)
+			continue;
+
+		return;
+	}
+
+	vfree_atomic(s);
+}
+
+static int scs_cleanup(unsigned int cpu)
+{
+	int i;
+	void **cache = per_cpu_ptr(scs_cache, cpu);
+
+	for (i = 0; i < SCS_CACHE_SIZE; i++) {
+		vfree(cache[i]);
+		cache[i] = NULL;
+	}
+
+	return 0;
+}
+
+void __init scs_init(void)
+{
+	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
+		scs_cleanup);
+}
+
+#else /* !CONFIG_SHADOW_CALL_STACK_VMAP */
+
+static struct kmem_cache *scs_cache;
+
+static inline void *scs_alloc(int node)
+{
+	return kmem_cache_alloc_node(scs_cache, GFP_SCS, node);
+}
+
+static inline void scs_free(void *s)
+{
+	kmem_cache_free(scs_cache, s);
+}
+
+void __init scs_init(void)
+{
+	scs_cache = kmem_cache_create("scs_cache", SCS_SIZE, SCS_SIZE,
+				0, NULL);
+	WARN_ON(!scs_cache);
+}
+
+#endif /* CONFIG_SHADOW_CALL_STACK_VMAP */
+
+static inline unsigned long *scs_magic(struct task_struct *tsk)
+{
+	return (unsigned long *)(__scs_base(tsk) + SCS_SIZE - sizeof(long));
+}
+
+static inline void scs_set_magic(struct task_struct *tsk)
+{
+	*scs_magic(tsk) = SCS_END_MAGIC;
+}
+
+void scs_task_init(struct task_struct *tsk)
+{
+	task_set_scs(tsk, NULL);
+}
+
+void scs_task_reset(struct task_struct *tsk)
+{
+	task_set_scs(tsk, __scs_base(tsk));
+}
+
+int scs_prepare(struct task_struct *tsk, int node)
+{
+	void *s;
+
+	s = scs_alloc(node);
+	if (!s)
+		return -ENOMEM;
+
+	task_set_scs(tsk, s);
+	scs_set_magic(tsk);
+
+	return 0;
+}
+
+bool scs_corrupted(struct task_struct *tsk)
+{
+	return *scs_magic(tsk) != SCS_END_MAGIC;
+}
+
+void scs_release(struct task_struct *tsk)
+{
+	void *s;
+
+	s = __scs_base(tsk);
+	if (!s)
+		return;
+
+	WARN_ON(scs_corrupted(tsk));
+
+	scs_task_init(tsk);
+	scs_free(s);
+}
-- 
2.24.0.rc0.303.g954a862665-goog

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