linux-kernel - Re: [RFC PATCH 5/5] selftest/x86: Add CET quick test

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Date:   Fri, 22 May 2020 11:28:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        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>
Subject: Re: [RFC PATCH 5/5] selftest/x86: Add CET quick test

On Thu, May 21, 2020 at 02:17:20PM -0700, Yu-cheng Yu wrote:

> +#pragma GCC push_options
> +#pragma GCC optimize ("O0")
> +void ibt_violation(void)
> +{
> +#ifdef __i386__
> +	asm volatile("lea 1f, %eax");
> +	asm volatile("jmp *%eax");
> +#else
> +	asm volatile("lea 1f, %rax");
> +	asm volatile("jmp *%rax");
> +#endif
> +	asm volatile("1:");
> +	result[test_id] = -1;
> +	test_id++;
> +	setcontext(&ucp);
> +}
> +
> +void shstk_violation(void)
> +{
> +#ifdef __i386__
> +	unsigned long x = 0;
> +
> +	((unsigned long *)&x)[2] = (unsigned long)stack_hacked;
> +#else
> +	unsigned long long x = 0;
> +
> +	((unsigned long long *)&x)[2] = (unsigned long)stack_hacked;
> +#endif
> +}
> +#pragma GCC pop_options

This is absolutely atrocious.

The #pragma like Kees already said just need to go. Also, there's
absolutely no clue what so ever what it attempts to achieve.

The __i386__ ifdeffery is horrible crap. Splitting an asm with #ifdef
like that is also horrible crap.

This is not how you write code.

Get asm/asm.h into userspace and then write something like:


void ibt_violation(void)
{
	asm volatile("lea  1f, %" _ASM_AX "\n\t"
		     "jmp  *%" _ASM_AX "\n\t"
		     "1:\n\t" ::: "a");

	WRITE_ONCE(result[test_id], -1);
	WRITE_ONCE(test_id, test_id+1);

	setcontext(&ucp);
}

void shstk_violation(void)
{
	unsigned long x = 0;

	WRITE_ONCE(x[2], stack_hacked);
}