[PATCH 1/9] x86/cet: Control protection exception handler

[Yu-cheng Yu]

A control protection exception is triggered when a control flow transfer
attempt violated shadow stack or indirect branch tracking constraints.
For example, the return address for a RET instruction differs from the
safe copy on the shadow stack; or a JMP instruction arrives at a non-
ENDBR instruction.

The control protection exception handler works in a similar way as the
general protection fault handler.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
 arch/x86/entry/entry_32.S    |  5 ++++
 arch/x86/entry/entry_64.S    |  2 +-
 arch/x86/include/asm/traps.h |  3 +++
 arch/x86/kernel/idt.c        |  1 +
 arch/x86/kernel/traps.c      | 61 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 71 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index bef8e2b202a8..14b63ef0d7d8 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1070,6 +1070,11 @@ ENTRY(general_protection)
 	jmp	common_exception
 END(general_protection)
 
+ENTRY(control_protection)
+	pushl	$do_control_protection
+	jmp	common_exception
+END(control_protection)
+
 #ifdef CONFIG_KVM_GUEST
 ENTRY(async_page_fault)
 	ASM_CLAC
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 3166b9674429..5230f705d229 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -999,7 +999,7 @@ idtentry spurious_interrupt_bug		do_spurious_interrupt_bug	has_error_code=0
 idtentry coprocessor_error		do_coprocessor_error		has_error_code=0
 idtentry alignment_check		do_alignment_check		has_error_code=1
 idtentry simd_coprocessor_error		do_simd_coprocessor_error	has_error_code=0
-
+idtentry control_protection		do_control_protection		has_error_code=1
 
 	/*
 	 * Reload gs selector with exception handling
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index 3de69330e6c5..5196050ff3d5 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -26,6 +26,7 @@ asmlinkage void invalid_TSS(void);
 asmlinkage void segment_not_present(void);
 asmlinkage void stack_segment(void);
 asmlinkage void general_protection(void);
+asmlinkage void control_protection(void);
 asmlinkage void page_fault(void);
 asmlinkage void async_page_fault(void);
 asmlinkage void spurious_interrupt_bug(void);
@@ -77,6 +78,7 @@ dotraplinkage void do_stack_segment(struct pt_regs *, long);
 dotraplinkage void do_double_fault(struct pt_regs *, long);
 #endif
 dotraplinkage void do_general_protection(struct pt_regs *, long);
+dotraplinkage void do_control_protection(struct pt_regs *, long);
 dotraplinkage void do_page_fault(struct pt_regs *, unsigned long);
 dotraplinkage void do_spurious_interrupt_bug(struct pt_regs *, long);
 dotraplinkage void do_coprocessor_error(struct pt_regs *, long);
@@ -142,6 +144,7 @@ enum {
 	X86_TRAP_AC,		/* 17, Alignment Check */
 	X86_TRAP_MC,		/* 18, Machine Check */
 	X86_TRAP_XF,		/* 19, SIMD Floating-Point Exception */
+	X86_TRAP_CP = 21,	/* 21 Control Protection Fault */
 	X86_TRAP_IRET = 32,	/* 32, IRET Exception */
 };
 
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index 2c3a1b4294eb..d00493709cec 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -85,6 +85,7 @@ static const __initconst struct idt_data def_idts[] = {
 	INTG(X86_TRAP_MF,		coprocessor_error),
 	INTG(X86_TRAP_AC,		alignment_check),
 	INTG(X86_TRAP_XF,		simd_coprocessor_error),
+	INTG(X86_TRAP_CP,		control_protection),
 
 #ifdef CONFIG_X86_32
 	TSKG(X86_TRAP_DF,		GDT_ENTRY_DOUBLEFAULT_TSS),
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 03f3d7695dac..4e8769a19aaf 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -577,6 +577,67 @@ do_general_protection(struct pt_regs *regs, long error_code)
 }
 NOKPROBE_SYMBOL(do_general_protection);
 
+static const char *control_protection_err[] =
+{
+	"near-ret",
+	"far-ret/iret",
+	"endbranch",
+	"rstorssp",
+	"setssbsy",
+	"unknown",
+};
+
+/*
+ * When a control protection exception occurs, send a signal
+ * to the responsible application.  Currently, control
+ * protection is only enabled for the user mode.  This
+ * exception should not come from the kernel mode.
+ */
+dotraplinkage void
+do_control_protection(struct pt_regs *regs, long error_code)
+{
+	struct task_struct *tsk;
+
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+	cond_local_irq_enable(regs);
+
+	tsk = current;
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK) &&
+	    !cpu_feature_enabled(X86_FEATURE_IBT)) {
+		goto exit;
+	}
+
+	if (!user_mode(regs)) {
+		tsk->thread.error_code = error_code;
+		tsk->thread.trap_nr = X86_TRAP_CP;
+		if (notify_die(DIE_TRAP, "control protection fault", regs,
+			       error_code, X86_TRAP_CP, SIGSEGV) != NOTIFY_STOP)
+			die("control protection fault", regs, error_code);
+		return;
+	}
+
+	tsk->thread.error_code = error_code;
+	tsk->thread.trap_nr = X86_TRAP_CP;
+
+	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
+	    printk_ratelimit()) {
+		unsigned int max_idx, err_idx;
+
+		max_idx = ARRAY_SIZE(control_protection_err) - 1;
+		err_idx = min((unsigned int)error_code - 1, max_idx);
+		pr_info("%s[%d] control protection ip:%lx sp:%lx error:%lx(%s)",
+			tsk->comm, task_pid_nr(tsk),
+			regs->ip, regs->sp, error_code,
+			control_protection_err[err_idx]);
+		print_vma_addr(" in ", regs->ip);
+		pr_cont("\n");
+	}
+
+exit:
+	force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
+}
+NOKPROBE_SYMBOL(do_control_protection);
+
 dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
 {
 #ifdef CONFIG_DYNAMIC_FTRACE
-- 
2.15.1