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Date:   Fri, 5 Feb 2021 10:29:10 -0800
From:   Kees Cook <keescook@chromium.org>
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Cc:     Borislav Petkov <bp@alien8.de>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
Subject: Re: [PATCH v19 06/25] x86/cet: Add control-protection fault handler
Message-ID: <202102051028.A10679FF@keescook>
References: <20210203225547.32221-1-yu-cheng.yu@intel.com>
 <20210203225547.32221-7-yu-cheng.yu@intel.com>
 <20210205135927.GH17488@zn.tnic>
 <2d829cba-784e-635a-e0c5-a7b334fa9b40@intel.com>
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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