linux-kernel - Re: [PATCH v21 06/26] x86/cet: Add control-protection fault handler

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Message-ID: <20210224161343.GE20344@zn.tnic>
Date:   Wed, 24 Feb 2021 17:13:43 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Yu-cheng Yu <yu-cheng.yu@...el.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@...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>,
        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>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>,
        Haitao Huang <haitao.huang@...el.com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH v21 06/26] x86/cet: Add control-protection fault handler

On Wed, Feb 17, 2021 at 02:27:10PM -0800, Yu-cheng Yu wrote:
> +/*
> + * 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);

Pls move that out of the function - those "static" qualifiers get missed
easily when inside a function.

> +	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;
> +
> +	/*
> +	 * Ratelimit to prevent log spamming.
> +	 */
> +	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> +	    __ratelimit(&rs)) {
> +		unsigned long ssp;
> +		int err;
> +
> +		err = array_index_nospec(error_code, ARRAY_SIZE(control_protection_err));

"err" as an automatic variable is confusing - we use those to denote
whether the function returned an error or not. Call yours "cpf_type" or
so.

> +
> +		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[err]);
> +		print_vma_addr(KERN_CONT " in ", regs->ip);
> +		pr_cont("\n");
> +	}
> +
> +	force_sig_fault(SIGSEGV, SEGV_CPERR,
> +			(void __user *)uprobe_get_trap_addr(regs));

Why is this calling an uprobes function?

Also, do not break that line even if it is longer than 80.

> +	cond_local_irq_disable(regs);
> +}
> +#endif
> +
>  static bool do_int3(struct pt_regs *regs)
>  {
>  	int res;
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index d2597000407a..1c2ea91284a0 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -231,7 +231,8 @@ typedef struct siginfo {
>  #define SEGV_ADIPERR	7	/* Precise MCD exception */
>  #define SEGV_MTEAERR	8	/* Asynchronous ARM MTE error */
>  #define SEGV_MTESERR	9	/* Synchronous ARM MTE exception */
> -#define NSIGSEGV	9
> +#define SEGV_CPERR	10	/* Control protection fault */
> +#define NSIGSEGV	10

I still don't see the patch adding this to the manpage of sigaction(2).

There's a git repo there: https://www.kernel.org/doc/man-pages/

and I'm pretty sure Michael takes patches.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette