Re: [PATCH v4 24/45] x86/ibt: Add IBT feature, MSR and #CP handling

[Andrew Cooper <Andrew.Cooper3@citrix.com>]

On 08/03/2022 15:30, Peter Zijlstra wrote:
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -592,6 +594,24 @@ 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);

This path wants to clear CR4.CET or MSR_S_CET, or both.

> +		return;
> +	}
> +}
> +
>  /*
>   * Some CPU features depend on higher CPUID levels, which may not always
>   * be available due to CPUID level capping or broken virtualization
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -210,6 +210,80 @@ DEFINE_IDTENTRY(exc_overflow)
>  	do_error_trap(regs, 0, "overflow", X86_TRAP_OF, SIGSEGV, 0, NULL);
>  }
>  
> +#ifdef CONFIG_X86_KERNEL_IBT
> +
> +static __ro_after_init bool ibt_fatal = true;
> +
> +void ibt_selftest_ip(void); /* code label defined in asm below */
> +
> +enum cp_error_code {
> +	CP_EC        = (1 << 15) - 1,
> +
> +	CP_RET       = 1,
> +	CP_IRET      = 2,
> +	CP_ENDBR     = 3,
> +	CP_RSTRORSSP = 4,
> +	CP_SETSSBSY  = 5,
> +
> +	CP_ENCL	     = 1 << 15,
> +};
> +
> +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> +{
> +	if (!cpu_feature_enabled(X86_FEATURE_IBT)) {
> +		pr_err("Unexpected #CP\n");
> +		BUG();
> +	}
> +
> +	if (WARN_ON_ONCE(user_mode(regs) || (error_code & CP_EC) != CP_ENDBR))
> +		return;

Is this wise?  #CP is a fault, so this will livelock.

user_mode() wants to turn into an oops.  Kernel probably wants to fold
into the Unexpected #CP path, but definitely print the error code.

~Andrew

> +
> +	if (unlikely(regs->ip == (unsigned long)ibt_selftest_ip)) {
> +		regs->ax = 0;
> +		return;
> +	}
> +
> +	pr_err("Missing ENDBR: %pS\n", (void *)instruction_pointer(regs));
> +	if (!ibt_fatal) {
> +		printk(KERN_DEFAULT CUT_HERE);
> +		__warn(__FILE__, __LINE__, (void *)regs->ip, TAINT_WARN, regs, NULL);
> +		return;
> +	}
> +	BUG();
> +}
> +
> +bool ibt_selftest(void)
> +{
> +	unsigned long ret;
> +
> +	asm ("	lea ibt_selftest_ip(%%rip), %%rax\n\t"
> +	     ANNOTATE_RETPOLINE_SAFE
> +	     "	jmp *%%rax\n\t"
> +	     "ibt_selftest_ip:\n\t"
> +	     UNWIND_HINT_FUNC
> +	     ANNOTATE_NOENDBR
> +	     "	nop\n\t"
> +
> +	     : "=a" (ret) : : "memory");
> +
> +	return !ret;
> +}
> +
> +static int __init ibt_setup(char *str)
> +{
> +	if (!strcmp(str, "off"))
> +		setup_clear_cpu_cap(X86_FEATURE_IBT);
> +
> +	if (!strcmp(str, "warn"))
> +		ibt_fatal = false;
> +
> +	return 1;
> +}
> +
> +__setup("ibt=", ibt_setup);
> +
> +#endif /* CONFIG_X86_KERNEL_IBT */
> +
>  #ifdef CONFIG_X86_F00F_BUG
>  void handle_invalid_op(struct pt_regs *regs)
>  #else
>
>