[PATCH v30 02/10] x86/cet/ibt: Add user-mode Indirect Branch Tracking support

[Yu-cheng Yu]

Introduce user-mode Indirect Branch Tracking (IBT) support.  Add routines
for the setup/disable of IBT.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
---
v28:
- When IBT feature is not present, make ibt_setup() return success,
  since this is a setup function.

v27:
- Change struct thread_shstk: ibt_enabled to ibt.
- Create a helper for set/clear bits of MSR_IA32_U_CET.
---
 arch/x86/include/asm/cet.h |  9 ++++++
 arch/x86/kernel/Makefile   |  1 +
 arch/x86/kernel/ibt.c      | 58 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 68 insertions(+)
 create mode 100644 arch/x86/kernel/ibt.c

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index fb32cb093ebb..bc04b2d3487d 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -11,6 +11,7 @@ struct thread_shstk {
 	u64	base;
 	u64	size;
 	u64	locked:1;
+	u64	ibt:1;
 };
 
 #ifdef CONFIG_X86_SHADOW_STACK
@@ -39,6 +40,14 @@ static inline int setup_signal_shadow_stack(int proc32, void __user *restorer) {
 static inline int restore_signal_shadow_stack(void) { return 0; }
 #endif
 
+#ifdef CONFIG_X86_IBT
+int ibt_setup(void);
+void ibt_disable(void);
+#else
+static inline int ibt_setup(void) { return 0; }
+static inline void ibt_disable(void) {}
+#endif
+
 #ifdef CONFIG_X86_SHADOW_STACK
 int prctl_cet(int option, u64 arg2);
 #else
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index 39e826b5cabd..cce07a920fec 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -152,6 +152,7 @@ obj-$(CONFIG_UNWINDER_GUESS)		+= unwind_guess.o
 obj-$(CONFIG_AMD_MEM_ENCRYPT)		+= sev.o
 obj-$(CONFIG_X86_SHADOW_STACK)		+= shstk.o
 obj-$(CONFIG_X86_SHADOW_STACK)		+= shstk.o cet_prctl.o
+obj-$(CONFIG_X86_IBT)			+= ibt.o
 ###
 # 64 bit specific files
 ifeq ($(CONFIG_X86_64),y)
diff --git a/arch/x86/kernel/ibt.c b/arch/x86/kernel/ibt.c
new file mode 100644
index 000000000000..4ab7af33b274
--- /dev/null
+++ b/arch/x86/kernel/ibt.c
@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * ibt.c - Intel Indirect Branch Tracking support
+ *
+ * Copyright (c) 2021, Intel Corporation.
+ * Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
+ */
+
+#include <linux/user.h>
+#include <asm/fpu/internal.h>
+#include <asm/fpu/xstate.h>
+#include <asm/fpu/types.h>
+#include <asm/msr.h>
+#include <asm/cet.h>
+
+static int ibt_set_clear_msr_bits(u64 set, u64 clear)
+{
+	u64 msr;
+	int r;
+
+	fpregs_lock();
+
+	if (test_thread_flag(TIF_NEED_FPU_LOAD))
+		fpregs_restore_userregs();
+
+	r = rdmsrl_safe(MSR_IA32_U_CET, &msr);
+	if (!r) {
+		msr = (msr & ~clear) | set;
+		r = wrmsrl_safe(MSR_IA32_U_CET, msr);
+	}
+
+	fpregs_unlock();
+
+	return r;
+}
+
+int ibt_setup(void)
+{
+	int r;
+
+	if (!cpu_feature_enabled(X86_FEATURE_IBT))
+		return 0;
+
+	r = ibt_set_clear_msr_bits(CET_ENDBR_EN | CET_NO_TRACK_EN, 0);
+	if (!r)
+		current->thread.shstk.ibt = 1;
+
+	return r;
+}
+
+void ibt_disable(void)
+{
+	if (!current->thread.shstk.ibt)
+		return;
+
+	ibt_set_clear_msr_bits(0, CET_ENDBR_EN);
+	current->thread.shstk.ibt = 0;
+}
-- 
2.21.0