scs: Add support for Clang's Shadow Call Stack (SCS)

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 in memory, which means an attacker capable reading
and writing arbitrary memory may be able to locate them and hijack
control flow by modifying the stacks.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
[will: Numerous cosmetic changes]
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 68 insertions, 0 deletions

diff --git a/include/linux/scs.h b/include/linux/scs.h
new file mode 100644
index 000000000000..3f3662621a27
--- /dev/null
+++ b/include/linux/scs.h
@@ -0,0 +1,68 @@
+/* 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 <linux/sizes.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		SZ_1K
+#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)
+
+void scs_init(void);
+int scs_prepare(struct task_struct *tsk, int node);
+void scs_release(struct task_struct *tsk);
+
+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;
+}
+
+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);
+}
+
+#else /* CONFIG_SHADOW_CALL_STACK */
+
+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 */