Re: [PATCH v2 07/39] x86/cet: Add user control-protection fault handler

[Edgecombe, Rick P]

On Wed, 2022-10-05 at 11:39 +0200, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:29:04PM -0700, Rick Edgecombe wrote:
> 
> > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> > index d62b2cb85cea..b7dde8730236 100644
> > --- a/arch/x86/kernel/traps.c
> > +++ b/arch/x86/kernel/traps.c
> > @@ -229,16 +223,74 @@ enum cp_error_code {
> >  	CP_ENCL	     = 1 << 15,
> >  };
> >  
> > -DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> > +#ifdef CONFIG_X86_SHADOW_STACK
> > +static const char * const control_protection_err[] = {
> > +	"unknown",
> > +	"near-ret",
> > +	"far-ret/iret",
> > +	"endbranch",
> > +	"rstorssp",
> > +	"setssbsy",
> > +};
> > +
> > +static DEFINE_RATELIMIT_STATE(cpf_rate,
> > DEFAULT_RATELIMIT_INTERVAL,
> > +			      DEFAULT_RATELIMIT_BURST);
> > +
> > +static void do_user_control_protection_fault(struct pt_regs *regs,
> > +					     unsigned long error_code)
> >  {
> > -	if (!cpu_feature_enabled(X86_FEATURE_IBT)) {
> > -		pr_err("Unexpected #CP\n");
> > -		BUG();
> > +	struct task_struct *tsk;
> > +	unsigned long ssp;
> > +
> > +	/* Read SSP before enabling interrupts. */
> > +	rdmsrl(MSR_IA32_PL3_SSP, ssp);
> > +
> > +	cond_local_irq_enable(regs);
> > +
> > +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> > +		WARN_ONCE(1, "User-mode control protection fault with
> > shadow support disabled\n");
> > +
> > +	tsk = current;
> > +	tsk->thread.error_code = error_code;
> > +	tsk->thread.trap_nr = X86_TRAP_CP;
> > +
> > +	/* Ratelimit to prevent log spamming. */
> > +	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> > +	    __ratelimit(&cpf_rate)) {
> > +		unsigned int cpec;
> > +
> > +		cpec = error_code & CP_EC;
> > +		if (cpec >= ARRAY_SIZE(control_protection_err))
> > +			cpec = 0;
> > +
> > +		pr_emerg("%s[%d] control protection ip:%lx sp:%lx
> > ssp:%lx error:%lx(%s)%s",
> > +			 tsk->comm, task_pid_nr(tsk),
> > +			 regs->ip, regs->sp, ssp, error_code,
> > +			 control_protection_err[cpec],
> > +			 error_code & CP_ENCL ? " in enclave" : "");
> > +		print_vma_addr(KERN_CONT " in ", regs->ip);
> > +		pr_cont("\n");
> >  	}
> >  
> > -	if (WARN_ON_ONCE(user_mode(regs) || (error_code & CP_EC) !=
> > CP_ENDBR))
> > -		return;
> 
> Why are you removing the (error_code & CP_EC) != CP_ENDBR check from
> the
> kernel handler?

Argh. It was accidentally removed with the user_mode() check. I'll fix
it.

> 
> > +	force_sig_fault(SIGSEGV, SEGV_CPERR, (void __user *)0);
> > +	cond_local_irq_disable(regs);
> > +}
> > +#else
> > +static void do_user_control_protection_fault(struct pt_regs *regs,
> > +					     unsigned long error_code)
> > +{
> > +	WARN_ONCE(1, "User-mode control protection fault with shadow
> > support disabled\n");
> > +}
> > +#endif
> > +
> > +#ifdef CONFIG_X86_KERNEL_IBT
> > +
> > +static __ro_after_init bool ibt_fatal = true;
> > +
> > +extern void ibt_selftest_ip(void); /* code label defined in asm
> > below */
> >  
> > +static void do_kernel_control_protection_fault(struct pt_regs
> > *regs)
> > +{
> >  	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_ip)) {
> >  		regs->ax = 0;
> >  		return;
> > @@ -283,9 +335,29 @@ static int __init ibt_setup(char *str)
> >  }
> >  
> >  __setup("ibt=", ibt_setup);
> > -
> > +#else
> > +static void do_kernel_control_protection_fault(struct pt_regs
> > *regs)
> > +{
> > +	WARN_ONCE(1, "Kernel-mode control protection fault with IBT
> > disabled\n");
> > +}
> >  #endif /* CONFIG_X86_KERNEL_IBT */
> >  
> > +#if defined(CONFIG_X86_KERNEL_IBT) ||
> > defined(CONFIG_X86_SHADOW_STACK)
> > +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> > +{
> > +	if (!cpu_feature_enabled(X86_FEATURE_IBT) &&
> > +	    !cpu_feature_enabled(X86_FEATURE_SHSTK)) {
> > +		pr_err("Unexpected #CP\n");
> > +		BUG();
> > +	}
> > +
> > +	if (user_mode(regs))
> > +		do_user_control_protection_fault(regs, error_code);
> > +	else
> > +		do_kernel_control_protection_fault(regs);
> 
> These function names are weirdly long, surely they can do without the
> _fault part at the very least. And as stated above, I would really
> like
> the kernel thing to retain the error_code argument.
> 

I can shorten them. Thanks.

> > +}
> > +#endif /* defined(CONFIG_X86_KERNEL_IBT) ||
> > defined(CONFIG_X86_SHADOW_STACK) */
> 
>