[RFC PATCH v2 2/3] scs: add support for dynamic shadow call stacks

[Ard Biesheuvel]

In order to allow arches to use code patching to conditionally emit the
shadow stack pushes and pops, rather than always taking the performance
hit even on CPUs that implement alternatives such as stack pointer
authentication on arm64, add a Kconfig symbol that can be set by the
arch to omit the SCS codegen itself, without otherwise affecting how
support code for SCS and compiler options (for register reservation, for
instance) are emitted.

Also, add a static key and some plumbing to omit the allocation of
shadow call stack for dynamic SCS configurations if SCS is disabled at
runtime.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 Makefile            |  2 ++
 arch/Kconfig        |  7 +++++++
 include/linux/scs.h | 10 ++++++++++
 kernel/scs.c        | 14 ++++++++++++--
 4 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index fa5112a0ec1b..a578fffc0337 100644
--- a/Makefile
+++ b/Makefile
@@ -882,8 +882,10 @@ LDFLAGS_vmlinux += --gc-sections
 endif
 
 ifdef CONFIG_SHADOW_CALL_STACK
+ifndef CONFIG_DYNAMIC_SCS
 CC_FLAGS_SCS	:= -fsanitize=shadow-call-stack
 KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
+endif
 export CC_FLAGS_SCS
 endif
 
diff --git a/arch/Kconfig b/arch/Kconfig
index 29b0167c088b..126caa75969a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -627,6 +627,13 @@ config SHADOW_CALL_STACK
 	  reading and writing arbitrary memory may be able to locate them
 	  and hijack control flow by modifying the stacks.
 
+config DYNAMIC_SCS
+	bool
+	help
+	  Set by the arch code if it relies on code patching to insert the
+	  shadow call stack push and pop instructions rather than on the
+	  compiler.
+
 config LTO
 	bool
 	help
diff --git a/include/linux/scs.h b/include/linux/scs.h
index 18122d9e17ff..4cc01f21b17a 100644
--- a/include/linux/scs.h
+++ b/include/linux/scs.h
@@ -53,6 +53,15 @@ static inline bool task_scs_end_corrupted(struct task_struct *tsk)
 	return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
 }
 
+DECLARE_STATIC_KEY_TRUE(dynamic_scs_enabled);
+
+static inline bool scs_is_enabled(void)
+{
+	if (!IS_ENABLED(CONFIG_DYNAMIC_SCS))
+		return true;
+	return static_branch_likely(&dynamic_scs_enabled);
+}
+
 #else /* CONFIG_SHADOW_CALL_STACK */
 
 static inline void *scs_alloc(int node) { return NULL; }
@@ -62,6 +71,7 @@ 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 void scs_release(struct task_struct *tsk) {}
 static inline bool task_scs_end_corrupted(struct task_struct *tsk) { return false; }
+static inline bool scs_is_enabled(void) { return false; }
 
 #endif /* CONFIG_SHADOW_CALL_STACK */
 
diff --git a/kernel/scs.c b/kernel/scs.c
index b7e1b096d906..8826794d2645 100644
--- a/kernel/scs.c
+++ b/kernel/scs.c
@@ -12,6 +12,10 @@
 #include <linux/vmalloc.h>
 #include <linux/vmstat.h>
 
+#ifdef CONFIG_DYNAMIC_SCS
+DEFINE_STATIC_KEY_TRUE(dynamic_scs_enabled);
+#endif
+
 static void __scs_account(void *s, int account)
 {
 	struct page *scs_page = vmalloc_to_page(s);
@@ -101,14 +105,20 @@ static int scs_cleanup(unsigned int cpu)
 
 void __init scs_init(void)
 {
+	if (!scs_is_enabled())
+		return;
 	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "scs:scs_cache", NULL,
 			  scs_cleanup);
 }
 
 int scs_prepare(struct task_struct *tsk, int node)
 {
-	void *s = scs_alloc(node);
+	void *s;
 
+	if (!scs_is_enabled())
+		return 0;
+
+	s = scs_alloc(node);
 	if (!s)
 		return -ENOMEM;
 
@@ -148,7 +158,7 @@ void scs_release(struct task_struct *tsk)
 {
 	void *s = task_scs(tsk);
 
-	if (!s)
+	if (!scs_is_enabled() || !s)
 		return;
 
 	WARN(task_scs_end_corrupted(tsk),
-- 
2.30.2