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

Yu-cheng Yu
2018-06-07T15:46:38Z

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@intel.com>
---
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