Re: [PATCH v3 04/37] x86/cpufeatures: Enable CET CR4 bit for shadow stack

[Borislav Petkov]

On Fri, Nov 04, 2022 at 03:35:31PM -0700, Rick Edgecombe wrote:
>  static __always_inline void setup_cet(struct cpuinfo_x86 *c)
>  {
> -	u64 msr = CET_ENDBR_EN;
> +	bool kernel_ibt = HAS_KERNEL_IBT && cpu_feature_enabled(X86_FEATURE_IBT);
> +	bool user_shstk;
> +	u64 msr = 0;
>  
> -	if (!HAS_KERNEL_IBT ||
> -	    !cpu_feature_enabled(X86_FEATURE_IBT))
> +	/*
> +	 * Enable user shadow stack only if the Linux defined user shadow stack
> +	 * cap was not cleared by command line.
> +	 */
> +	user_shstk = cpu_feature_enabled(X86_FEATURE_SHSTK) &&
> +		     IS_ENABLED(CONFIG_X86_USER_SHADOW_STACK) &&
> +		     !test_bit(X86_FEATURE_USER_SHSTK, (unsigned long *)cpu_caps_cleared);

Huh, why poke at cpu_caps_cleared? 

Look below:

> +	if (!kernel_ibt && !user_shstk)
>  		return;
>  
> +	if (user_shstk)
> +		set_cpu_cap(c, X86_FEATURE_USER_SHSTK);
> +
> +	if (kernel_ibt)
> +		msr = CET_ENDBR_EN;
> +
>  	wrmsrl(MSR_IA32_S_CET, msr);
>  	cr4_set_bits(X86_CR4_CET);
>  
> -	if (!ibt_selftest()) {
> +	if (kernel_ibt && !ibt_selftest()) {
>  		pr_err("IBT selftest: Failed!\n");
>  		setup_clear_cpu_cap(X86_FEATURE_IBT);
>  		return;
>  	}
>  }
> +#else /* CONFIG_X86_CET */
> +static inline void setup_cet(struct cpuinfo_x86 *c) {}
> +#endif
>  
>  __noendbr void cet_disable(void)
>  {
> -	if (cpu_feature_enabled(X86_FEATURE_IBT))
> -		wrmsrl(MSR_IA32_S_CET, 0);
> +	if (!(cpu_feature_enabled(X86_FEATURE_IBT) ||
> +	      cpu_feature_enabled(X86_FEATURE_SHSTK)))
> +		return;
> +
> +	wrmsrl(MSR_IA32_S_CET, 0);
> +	wrmsrl(MSR_IA32_U_CET, 0);

Here you need to do

	setup_clear_cpu_cap(X86_FEATURE_IBT);
	setup_clear_cpu_cap(X86_FEATURE_SHSTK);

and then the cpu_feature_enabled() test above alone should suffice.

But, before you do that, I'd like to ask you to update your patchset
ontop of tip/master because the conflicts are getting non-trivial. This
one doesn't even want to apply with a large fuzz:

$ patch -p1 --dry-run -F20 -i /tmp/new
checking file arch/x86/kernel/cpu/common.c
Hunk #1 FAILED at 596.
1 out of 1 hunk FAILED

Thx.

-- 
Regards/Gruss,
    Boris.

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