Re: [PATCH v2 07/39] x86/cet: Add user control-protection fault handler

["H. Peter Anvin" <hpa@zytor.com>]

On October 3, 2022 3:51:59 PM PDT, Andy Lutomirski <luto@kernel.org> wrote:
>On 9/29/22 15:29, Rick Edgecombe wrote:
>> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>> 
>
>> +static void do_user_control_protection_fault(struct pt_regs *regs,
>> +					     unsigned long error_code)
>>   {
>> -	if (!cpu_feature_enabled(X86_FEATURE_IBT)) {
>> -		pr_err("Unexpected #CP\n");
>> -		BUG();
>> +	struct task_struct *tsk;
>> +	unsigned long ssp;
>> +
>> +	/* Read SSP before enabling interrupts. */
>> +	rdmsrl(MSR_IA32_PL3_SSP, ssp); > +
>> +	cond_local_irq_enable(regs);
>
>I feel like I'm missing something.  Either PL3_SSL is context switched correctly and reading it with IRQs off is useless, or it's not context switched, and I'm very confused.
>
>Please either improve the comment or move it after the cond_local_irq_enable().
>
>--Andy
>
>> +
>> +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
>> +		WARN_ONCE(1, "User-mode control protection fault with shadow 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(&cpf_rate)) {
>> +		unsigned int cpec;
>> +
>> +		cpec = error_code & CP_EC;
>> +		if (cpec >= ARRAY_SIZE(control_protection_err))
>> +			cpec = 0;
>> +
>> +		pr_emerg("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)%s",
>> +			 tsk->comm, task_pid_nr(tsk),
>> +			 regs->ip, regs->sp, ssp, error_code,
>> +			 control_protection_err[cpec],
>> +			 error_code & CP_ENCL ? " in enclave" : "");
>> +		print_vma_addr(KERN_CONT " in ", regs->ip);
>> +		pr_cont("\n");
>>   	}
>>   -	if (WARN_ON_ONCE(user_mode(regs) || (error_code & CP_EC) != CP_ENDBR))
>> -		return;
>> +	force_sig_fault(SIGSEGV, SEGV_CPERR, (void __user *)0);
>> +	cond_local_irq_disable(regs);
>> +}
>> +#else
>> +static void do_user_control_protection_fault(struct pt_regs *regs,
>> +					     unsigned long error_code)
>> +{
>> +	WARN_ONCE(1, "User-mode control protection fault with shadow support disabled\n");
>> +}
>> +#endif
>> +
>> +#ifdef CONFIG_X86_KERNEL_IBT
>> +
>> +static __ro_after_init bool ibt_fatal = true;
>> +
>> +extern void ibt_selftest_ip(void); /* code label defined in asm below */
>>   +static void do_kernel_control_protection_fault(struct pt_regs *regs)
>> +{
>>   	if (unlikely(regs->ip == (unsigned long)&ibt_selftest_ip)) {
>>   		regs->ax = 0;
>>   		return;
>> @@ -283,9 +335,29 @@ static int __init ibt_setup(char *str)
>>   }
>>     __setup("ibt=", ibt_setup);
>> -
>> +#else
>> +static void do_kernel_control_protection_fault(struct pt_regs *regs)
>> +{
>> +	WARN_ONCE(1, "Kernel-mode control protection fault with IBT disabled\n");
>> +}
>>   #endif /* CONFIG_X86_KERNEL_IBT */
>>   +#if defined(CONFIG_X86_KERNEL_IBT) || defined(CONFIG_X86_SHADOW_STACK)
>> +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
>> +{
>> +	if (!cpu_feature_enabled(X86_FEATURE_IBT) &&
>> +	    !cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>> +		pr_err("Unexpected #CP\n");
>> +		BUG();
>> +	}
>> +
>> +	if (user_mode(regs))
>> +		do_user_control_protection_fault(regs, error_code);
>> +	else
>> +		do_kernel_control_protection_fault(regs);
>> +}
>> +#endif /* defined(CONFIG_X86_KERNEL_IBT) || defined(CONFIG_X86_SHADOW_STACK) */
>> +
>>   #ifdef CONFIG_X86_F00F_BUG
>>   void handle_invalid_op(struct pt_regs *regs)
>>   #else
>> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
>> index 0ed2e487a693..57faa287163f 100644
>> --- a/arch/x86/xen/enlighten_pv.c
>> +++ b/arch/x86/xen/enlighten_pv.c
>> @@ -628,7 +628,7 @@ static struct trap_array_entry trap_array[] = {
>>   	TRAP_ENTRY(exc_coprocessor_error,		false ),
>>   	TRAP_ENTRY(exc_alignment_check,			false ),
>>   	TRAP_ENTRY(exc_simd_coprocessor_error,		false ),
>> -#ifdef CONFIG_X86_KERNEL_IBT
>> +#if defined(CONFIG_X86_KERNEL_IBT) || defined(CONFIG_X86_SHADOW_STACK)
>>   	TRAP_ENTRY(exc_control_protection,		false ),
>>   #endif
>>   };
>> diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
>> index 6b4fdf6b9542..e45ff6300c7d 100644
>> --- a/arch/x86/xen/xen-asm.S
>> +++ b/arch/x86/xen/xen-asm.S
>> @@ -148,7 +148,7 @@ xen_pv_trap asm_exc_page_fault
>>   xen_pv_trap asm_exc_spurious_interrupt_bug
>>   xen_pv_trap asm_exc_coprocessor_error
>>   xen_pv_trap asm_exc_alignment_check
>> -#ifdef CONFIG_X86_KERNEL_IBT
>> +#if defined(CONFIG_X86_KERNEL_IBT) || defined(CONFIG_X86_SHADOW_STACK)
>>   xen_pv_trap asm_exc_control_protection
>>   #endif
>>   #ifdef CONFIG_X86_MCE
>> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
>> index ffbe4cec9f32..0f52d0ac47c5 100644
>> --- a/include/uapi/asm-generic/siginfo.h
>> +++ b/include/uapi/asm-generic/siginfo.h
>> @@ -242,7 +242,8 @@ 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
>>     /*
>>    * SIGBUS si_codes
>

Could something change the value under a switched-out thread, though?