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Subject[PATCH 06/18] add support for Clang's Shadow Call Stack (SCS)
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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

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
---
Makefile | 6 ++
arch/Kconfig | 39 ++++++++
include/linux/compiler-clang.h | 2 +
include/linux/compiler_types.h | 4 +
include/linux/scs.h | 88 ++++++++++++++++++
init/init_task.c | 6 ++
init/main.c | 3 +
kernel/Makefile | 1 +
kernel/fork.c | 9 ++
kernel/sched/core.c | 2 +
kernel/sched/sched.h | 1 +
kernel/scs.c | 162 +++++++++++++++++++++++++++++++++
12 files changed, 323 insertions(+)
create mode 100644 include/linux/scs.h
create mode 100644 kernel/scs.c

diff --git a/Makefile b/Makefile
index ffd7a912fc46..e401fa500f62 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
+KBUILD_CFLAGS += -fsanitize=shadow-call-stack
+DISABLE_SCS := -fno-sanitize=shadow-call-stack
+export DISABLE_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..a222adda8130 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -521,6 +521,45 @@ 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
+ def_bool n
+ 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.
+
+choice
+ prompt "Return-oriented programming (ROP) protection"
+ default ROP_PROTECTION_NONE
+ help
+ This option controls kernel protections against return-oriented
+ programming (ROP) attacks.
+
+config ROP_PROTECTION_NONE
+ bool "None"
+
+config SHADOW_CALL_STACK
+ bool "Clang Shadow Call Stack"
+ depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
+ depends on CC_IS_CLANG && CLANG_VERSION >= 70000
+ 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
+
+endchoice
+
config HAVE_ARCH_WITHIN_STACK_FRAMES
bool
help
diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index 333a6695a918..9af08391f205 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -42,3 +42,5 @@
* compilers, like ICC.
*/
#define barrier() __asm__ __volatile__("" : : : "memory")
+
+#define __noscs __attribute__((no_sanitize("shadow-call-stack")))
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..dfbd80faa528
--- /dev/null
+++ b/include/linux/scs.h
@@ -0,0 +1,88 @@
+/* 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
+
+#ifdef CONFIG_SHADOW_CALL_STACK_VMAP
+# define SCS_SIZE PAGE_SIZE
+#else
+# define SCS_SIZE 1024
+#endif
+
+#define SCS_GFP (GFP_KERNEL | __GFP_ZERO)
+
+extern unsigned long init_shadow_call_stack[];
+
+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_set_init_magic(struct task_struct *tsk);
+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_set_init_magic(struct task_struct *tsk)
+{
+}
+
+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..5e55ff45bbbf 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 +185,11 @@ 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);
+#endif
+
/*
* Initial thread structure. Alignment of this is handled by a special
* linker map entry.
diff --git a/init/main.c b/init/main.c
index 91f6ebb30ef0..fb8bcdd729b9 100644
--- a/init/main.c
+++ b/init/main.c
@@ -93,6 +93,7 @@
#include <linux/rodata_test.h>
#include <linux/jump_label.h>
#include <linux/mem_encrypt.h>
+#include <linux/scs.h>

#include <asm/io.h>
#include <asm/bugs.h>
@@ -578,6 +579,8 @@ asmlinkage __visible void __init start_kernel(void)
char *after_dashes;

set_task_stack_end_magic(&init_task);
+ scs_set_init_magic(&init_task);
+
smp_setup_processor_id();
debug_objects_early_init();

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..47324e8d313b
--- /dev/null
+++ b/kernel/scs.c
@@ -0,0 +1,162 @@
+// 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>
+
+#define SCS_END_MAGIC 0xaf0194819b1635f6UL
+
+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;
+ }
+ }
+
+ return __vmalloc_node_range(SCS_SIZE, SCS_SIZE,
+ VMALLOC_START, VMALLOC_END,
+ SCS_GFP, 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, SCS_GFP, 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));
+}
+
+void scs_set_init_magic(struct task_struct *tsk)
+{
+ scs_save(tsk);
+ scs_set_magic(tsk);
+ scs_load(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);
+}
--
2.23.0.866.gb869b98d4c-goog
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