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

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Message-ID: <202102051028.A10679FF@keescook>
Date:   Fri, 5 Feb 2021 10:29:10 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, 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@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        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>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v19 06/25] x86/cet: Add control-protection fault handler

On Fri, Feb 05, 2021 at 10:00:21AM -0800, Yu, Yu-cheng wrote:
> On 2/5/2021 5:59 AM, Borislav Petkov wrote:
> > On Wed, Feb 03, 2021 at 02:55:28PM -0800, Yu-cheng Yu wrote:
> > > +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)) {
> > 
> > I can't find it written down anywhere why the ratelimiting is needed at
> > all?
> > 
> 
> The ratelimit here is only for #CP, and its rate is not counted together
> with other types of faults.  If a task gets here, it will exit.  The only
> condition the ratelimit will trigger is when multiple tasks hit #CP at once,
> which is unlikely.  Are you suggesting that we do not need the ratelimit
> here?

Since this is a potentially unprivileged-userspace-triggerable
condition, I tend to prefer having a ratelimit. I don't feel _strongly_
about it, but I find it better to be defensive against log spamming
(whether malicious or accidental).

-- 
Kees Cook