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

[Yu, Yu-cheng]

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!

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