linux-kernel - Re: [RFC PATCH v4 06/27] x86/cet: Control protection exception handler

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Date:   Wed, 03 Oct 2018 09:11:34 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Eugene Syromiatnikov <esyr@...hat.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...capital.net>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>
Subject: Re: [RFC PATCH v4 06/27] x86/cet: Control protection exception
 handler

On Wed, 2018-10-03 at 12:39 +0200, Eugene Syromiatnikov wrote:
> On Fri, Sep 21, 2018 at 08:03:30AM -0700, Yu-cheng Yu wrote:
> > +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");
> > +	if (notify_die(DIE_TRAP, "control protection fault", regs,
> > +		       error_code, X86_TRAP_CP, SIGSEGV) == NOTIFY_STOP)
> > +		return;
> > +	cond_local_irq_enable(regs);
> > +
> > +	if (!user_mode(regs))
> > +		die("kernel control protection fault", regs, error_code);
> > +
> > +	if (!static_cpu_has(X86_FEATURE_SHSTK) &&
> > +	    !static_cpu_has(X86_FEATURE_IBT))
> > +		WARN_ONCE(1, "CET is disabled but got control "
> > +			  "protection fault\n");
> > +
> > +	tsk = current;
> > +	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_err;
> > +
> > +		max_err = ARRAY_SIZE(control_protection_err) - 1;
> > +		if ((error_code < 0) || (error_code > max_err))
> > +			error_code = 0;
> > +		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[error_code]);
> > +		print_vma_addr(KERN_CONT " in ", regs->ip);
> > +		pr_cont("\n");
> > +	}
> > +
> > +	force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
> 
> That way, no information is provided to userspace (both application and
> debugger), which is rather unfortunate. It would be nice if a new SEGV_*
> code was added at least, and CET error (with error code constant provided
> in UAPI) is passed via si_errno. (Having ip/sp/*ssp would be even
> better, but I'm not exactly sure about ramifications of providing this
> kind of information to user space).

Ok, I will add that.

Yu-cheng