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Date:   Thu, 24 Feb 2022 17:09:32 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Thu, Feb 24, 2022 at 03:51:56PM +0100, Peter Zijlstra wrote:
> [...]
> @@ -438,7 +439,8 @@ static __always_inline void setup_umip(s
>  
>  /* These bits should not change their value after CPU init is finished. */
>  static const unsigned long cr4_pinned_mask =
> -	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP | X86_CR4_FSGSBASE;
> +	X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
> +	X86_CR4_FSGSBASE | X86_CR4_CET;

Thanks!

>  static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
>  static unsigned long cr4_pinned_bits __ro_after_init;
>  
> @@ -592,6 +594,29 @@ static __init int setup_disable_pku(char
>  __setup("nopku", setup_disable_pku);
>  #endif /* CONFIG_X86_64 */
>  
> +static __always_inline void setup_cet(struct cpuinfo_x86 *c)
> +{
> +	u64 msr = CET_ENDBR_EN;
> +
> +	if (!HAS_KERNEL_IBT ||
> +	    !cpu_feature_enabled(X86_FEATURE_IBT))
> +		return;
> +
> +	wrmsrl(MSR_IA32_S_CET, msr);
> +	cr4_set_bits(X86_CR4_CET);
> +
> +	if (!ibt_selftest()) {
> +		pr_err("IBT selftest: Failed!\n");
> +		setup_clear_cpu_cap(X86_FEATURE_IBT);
> +	}

For easy boot-output testing, I'd love to see something like:

	} else {
		pr_info("CET detected: Indirect Branch Tracking enabled.\n")
	}

or maybe:
		pr_info("CET detected: Indirect Branch Tracking is %s.\n",
			ibt_fatal ? "enforced" : "warning only");

> [...]
> +bool ibt_selftest(void)
> +{
> +	unsigned long ret;
> +
> +	asm ("1: lea 2f(%%rip), %%rax\n\t"
> +	     ANNOTATE_RETPOLINE_SAFE
> +	     "   jmp *%%rax\n\t"
> +	     ASM_REACHABLE
> +	     ANNOTATE_NOENDBR
> +	     "2: nop\n\t"
> +
> +	     /* unsigned ibt_selftest_ip = 2b */
> +	     ".pushsection .rodata,\"a\"\n\t"
> +	     ".align 8\n\t"
> +	     ".type ibt_selftest_ip, @object\n\t"
> +	     ".size ibt_selftest_ip, 8\n\t"
> +	     "ibt_selftest_ip:\n\t"
> +	     ".quad 2b\n\t"
> +	     ".popsection\n\t"
> +
> +	     : "=a" (ret) : : "memory");
> +
> +	return !ret;
> +}

I did something like this for LKDTM, but I realize it depends on having no
frame pointer, and is likely x86-specific too, as I think arm64's function
preamble is responsible for pushing the return address on the stack:


static volatile int lkdtm_just_count;

/* Function taking one argument, returning int. */
static noinline void *lkdtm_just_return(void)
{
	/* Do something after preamble but before label. */
	lkdtm_just_count++;

yolo:
	{
		void *right_here = &&yolo;

		OPTIMIZER_HIDE_VAR(right_here);
		return right_here;
	}
}
/*
 * This tries to call an indirect function in the middle.
 */
void lkdtm_CFI_FORWARD_ENTRY(void)
{
	/*
	 * Matches lkdtm_increment_void()'s prototype, but not
	 * lkdtm_increment_int()'s prototype.
	 */
	void * (*func)(void);

	func = lkdtm_just_return;
	pr_info("Calling actual function entry point %px ...\n", func);
	func = func();

	pr_info("Calling middle of function %px ...\n", func);
	func = func();

	pr_err("FAIL: survived non-entry point call!\n");
#ifdef CONFIG_X86
	pr_expected_config(CONFIG_X86_BTI);
#endif
}

-- 
Kees Cook

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