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>

4 files changed, 77 insertions, 0 deletions

diff --git a/kernel/Makefile b/kernel/Makefile
index 4cb4130ced32..c332eb9d4841 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -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/
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 8c700f881d92..f6339f9d232d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -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
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a2fbf98fd6f..934e03cfaec7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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
diff --git a/kernel/scs.c b/kernel/scs.c
new file mode 100644
index 000000000000..38f8f31c9451
--- /dev/null
+++ b/kernel/scs.c
@@ -0,0 +1,65 @@
+// 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 = 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 = task_scs(tsk);
+
+	if (!s)
+		return;
+
+	WARN(scs_corrupted(tsk), "corrupted shadow stack detected when freeing task\n");
+	scs_free(s);
+}