# Re: \[PATCH v19 06/25\] x86/cet: Add control-protection fault handler

- From: Yu, Yu-cheng \<hidden\>
- Date: 2021-02-05 00:11:12 
- List: linux-doc
- Page: https://ratatoskr.run/linux-doc/2021/02/6259336
- Thread: https://ratatoskr.run/linux-doc/2021/02/6257763/t.md

```text
On 2/4/2021 12:09 PM, Kees Cook wrote:
> On Wed, Feb 03, 2021 at 02:55:28PM -0800, Yu-cheng Yu wrote:

[...]

>> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
>> index 7f5aec758f0e..f5354c35df32 100644
>> --- a/arch/x86/kernel/traps.c
>> +++ b/arch/x86/kernel/traps.c
>> @@ -606,6 +606,66 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>>   	cond_local_irq_disable(regs);
>>   }
>>   
>> +#ifdef CONFIG_X86_CET
>> +static const char * const control_protection_err[] = {
>> +	"unknown",
>> +	"near-ret",
>> +	"far-ret/iret",
>> +	"endbranch",
>> +	"rstorssp",
>> +	"setssbsy",
>> +};
>> +
>> +/*
>> + * When a control protection exception occurs, send a signal to the responsible
>> + * application.  Currently, control protection is only enabled for user mode.
>> + * This exception should not come from kernel mode.
>> + */
>> +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
>> +{
>> +	static DEFINE_RATELIMIT_STATE(rs, DEFAULT_RATELIMIT_INTERVAL,
>> +				      DEFAULT_RATELIMIT_BURST);
>> +	struct task_struct *tsk;
>> +
>> +	if (!user_mode(regs)) {
>> +		pr_emerg("PANIC: unexpected kernel control protection fault\n");
>> +		die("kernel control protection fault", regs, error_code);
>> +		panic("Machine halted.");
>> +	}
>> +
>> +	cond_local_irq_enable(regs);
>> +
>> +	if (!boot_cpu_has(X86_FEATURE_CET))
>> +		WARN_ONCE(1, "Control protection fault with CET support disabled\n");
>> +
>> +	tsk = current;
>> +	tsk->thread.error_code = error_code;
>> +	tsk->thread.trap_nr = X86_TRAP_CP;
>> +
>> +	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
>> +	    __ratelimit(&rs)) {
>> +		unsigned int max_err;
>> +		unsigned long ssp;
>> +
>> +		max_err = ARRAY_SIZE(control_protection_err) - 1;
>> +		if (error_code < 0 || error_code > max_err)
>> +			error_code = 0;
>
> Do you want to mask the error_code here before printing its value?
>
>> +
>> +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
>> +		pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)",
>> +			 tsk->comm, task_pid_nr(tsk),
>> +			 regs->ip, regs->sp, ssp, error_code,
>> +			 control_protection_err[error_code]);
>
> Instead, you could clamp error_code to ARRAY_SIZE(control_protection_err),
> and add another "unknown" to the end of the strings:
>
> 	control_protection_err[
> 		array_index_nospec(error_code,
> 				   ARRAY_SIZE(control_protection_err))]
>
> Everything else looks good.
>

I will update it.  Thanks!

[...]
```