@@ -866,6 +866,12 @@ 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)
@@ -533,6 +533,30 @@ 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
+ bool "Clang Shadow Call Stack"
+ depends on CC_IS_CLANG && 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 in
+ 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 in memory, which means an attacker capable of
+ reading and writing arbitrary memory may be able to locate them
+ and hijack control flow by modifying the stacks.
+
config HAVE_ARCH_WITHIN_STACK_FRAMES
bool
help
@@ -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
@@ -193,6 +193,10 @@ 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
new file mode 100644
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Shadow Call Stack support.
+ *
+ * Copyright (C) 2019 Google LLC
+ */
+
+#ifndef _LINUX_SCS_H
+#define _LINUX_SCS_H
+
+#include <linux/gfp.h>
+#include <linux/poison.h>
+#include <linux/sched.h>
+#include <asm/page.h>
+
+#ifdef CONFIG_SHADOW_CALL_STACK
+
+/*
+ * In testing, 1 KiB shadow stack size (i.e. 128 stack frames on a 64-bit
+ * architecture) provided ~40% safety margin on stack usage while keeping
+ * memory allocation overhead reasonable.
+ */
+#define SCS_SIZE 1024UL
+#define GFP_SCS (GFP_KERNEL | __GFP_ZERO)
+
+/* An illegal pointer value to mark the end of the shadow stack. */
+#define SCS_END_MAGIC (0x5f6UL + POISON_POINTER_DELTA)
+
+#define task_scs(tsk) (task_thread_info(tsk)->scs_base)
+#define task_scs_offset(tsk) (task_thread_info(tsk)->scs_offset)
+
+extern void scs_init(void);
+
+static inline void scs_task_reset(struct task_struct *tsk)
+{
+ /*
+ * Reset the shadow stack to the base address in case the task
+ * is reused.
+ */
+ task_scs_offset(tsk) = 0;
+}
+
+extern int scs_prepare(struct task_struct *tsk, int node);
+
+static inline unsigned long *__scs_magic(void *s)
+{
+ return (unsigned long *)(s + SCS_SIZE) - 1;
+}
+
+static inline bool scs_corrupted(struct task_struct *tsk)
+{
+ unsigned long *magic = __scs_magic(task_scs(tsk));
+
+ return (task_scs_offset(tsk) >= SCS_SIZE - 1 ||
+ READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC);
+}
+
+extern void scs_release(struct task_struct *tsk);
+
+#else /* CONFIG_SHADOW_CALL_STACK */
+
+#define task_scs(tsk) NULL
+
+static inline void scs_init(void) {}
+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 */
@@ -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>
@@ -50,6 +51,13 @@ static struct sighand_struct init_sighand = {
.signalfd_wqh = __WAIT_QUEUE_HEAD_INITIALIZER(init_sighand.signalfd_wqh),
};
+#ifdef CONFIG_SHADOW_CALL_STACK
+unsigned long init_shadow_call_stack[SCS_SIZE / sizeof(long)]
+ __init_task_data = {
+ [(SCS_SIZE / sizeof(long)) - 1] = SCS_END_MAGIC
+};
+#endif
+
/*
* Set up the first task table, touch at your own risk!. Base=0,
* limit=0x1fffff (=2MB)
@@ -103,6 +103,7 @@ 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/
@@ -94,6 +94,7 @@
#include <linux/thread_info.h>
#include <linux/stackleak.h>
#include <linux/kasan.h>
+#include <linux/scs.h>
#include <asm/pgtable.h>
#include <asm/pgalloc.h>
@@ -456,6 +457,8 @@ 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,
@@ -840,6 +843,8 @@ void __init fork_init(void)
NULL, free_vm_stack_cache);
#endif
+ scs_init();
+
lockdep_init_task(&init_task);
uprobes_init();
}
@@ -899,6 +904,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
if (err)
goto free_stack;
+ err = scs_prepare(tsk, node);
+ if (err)
+ goto free_stack;
+
#ifdef CONFIG_SECCOMP
/*
* We must handle setting up seccomp filters once we're under
@@ -11,6 +11,7 @@
#include <linux/nospec.h>
#include <linux/kcov.h>
+#include <linux/scs.h>
#include <asm/switch_to.h>
#include <asm/tlb.h>
@@ -6040,6 +6041,7 @@ void init_idle(struct task_struct *idle, int cpu)
idle->se.exec_start = sched_clock();
idle->flags |= PF_IDLE;
+ scs_task_reset(idle);
kasan_unpoison_task_stack(idle);
#ifdef CONFIG_SMP
new file mode 100644
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Shadow Call Stack support.
+ *
+ * Copyright (C) 2019 Google LLC
+ */
+
+#include <linux/kasan.h>
+#include <linux/scs.h>
+#include <linux/slab.h>
+#include <asm/scs.h>
+
+static struct kmem_cache *scs_cache;
+
+static void *scs_alloc(int node)
+{
+ void *s;
+
+ s = kmem_cache_alloc_node(scs_cache, GFP_SCS, node);
+ if (s) {
+ *__scs_magic(s) = SCS_END_MAGIC;
+ /*
+ * Poison the allocation to catch unintentional accesses to
+ * the shadow stack when KASAN is enabled.
+ */
+ kasan_poison_object_data(scs_cache, s);
+ }
+
+ return s;
+}
+
+static void scs_free(void *s)
+{
+ kasan_unpoison_object_data(scs_cache, s);
+ kmem_cache_free(scs_cache, s);
+}
+
+void __init scs_init(void)
+{
+ scs_cache = kmem_cache_create("scs_cache", SCS_SIZE, 0, 0, NULL);
+}
+
+int scs_prepare(struct task_struct *tsk, int node)
+{
+ void *s;
+
+ s = scs_alloc(node);
+ if (!s)
+ return -ENOMEM;
+
+ task_scs(tsk) = s;
+ task_scs_offset(tsk) = 0;
+
+ return 0;
+}
+
+void scs_release(struct task_struct *tsk)
+{
+ void *s;
+
+ s = task_scs(tsk);
+ if (!s)
+ return;
+
+ WARN_ON(scs_corrupted(tsk));
+
+ scs_free(s);
+}