Re: [RFC PATCH v9 04/27] x86/cet: Add control-protection fault handler

[Kees Cook]

On Wed, Feb 05, 2020 at 10:19:12AM -0800, Yu-cheng Yu wrote:
> A control-protection fault is triggered when a control-flow transfer
> attempt violates Shadow Stack or Indirect Branch Tracking constraints.
> For example, the return address for a RET instruction differs from the copy
> on the Shadow Stack; or an indirect JMP instruction, without the NOTRACK
> prefix, arrives at a non-ENDBR opcode.
> 
> The control-protection fault handler works in a similar way as the general
> protection fault handler.  It provides the si_code SEGV_CPERR to the signal
> handler.
> 
> v9:
> - Add Shadow Stack pointer to the fault printout.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  arch/x86/entry/entry_64.S          |  2 +-
>  arch/x86/include/asm/traps.h       |  3 ++
>  arch/x86/kernel/idt.c              |  4 ++
>  arch/x86/kernel/signal_compat.c    |  2 +-
>  arch/x86/kernel/traps.c            | 59 ++++++++++++++++++++++++++++++
>  include/uapi/asm-generic/siginfo.h |  3 +-
>  6 files changed, 70 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 76942cbd95a1..6ca77312d008 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1034,7 +1034,7 @@ idtentry spurious_interrupt_bug		do_spurious_interrupt_bug	has_error_code=0
>  idtentry coprocessor_error		do_coprocessor_error		has_error_code=0
>  idtentry alignment_check		do_alignment_check		has_error_code=1
>  idtentry simd_coprocessor_error		do_simd_coprocessor_error	has_error_code=0
> -
> +idtentry control_protection		do_control_protection		has_error_code=1
>  
>  	/*
>  	 * Reload gs selector with exception handling
> diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
> index ffa0dc8a535e..7ac26bbd0bef 100644
> --- a/arch/x86/include/asm/traps.h
> +++ b/arch/x86/include/asm/traps.h
> @@ -26,6 +26,7 @@ asmlinkage void invalid_TSS(void);
>  asmlinkage void segment_not_present(void);
>  asmlinkage void stack_segment(void);
>  asmlinkage void general_protection(void);
> +asmlinkage void control_protection(void);
>  asmlinkage void page_fault(void);
>  asmlinkage void async_page_fault(void);
>  asmlinkage void spurious_interrupt_bug(void);
> @@ -84,6 +85,7 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s);
>  void __init trap_init(void);
>  #endif
>  dotraplinkage void do_general_protection(struct pt_regs *regs, long error_code);
> +dotraplinkage void do_control_protection(struct pt_regs *regs, long error_code);
>  dotraplinkage void do_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address);
>  dotraplinkage void do_spurious_interrupt_bug(struct pt_regs *regs, long error_code);
>  dotraplinkage void do_coprocessor_error(struct pt_regs *regs, long error_code);
> @@ -154,6 +156,7 @@ enum {
>  	X86_TRAP_AC,		/* 17, Alignment Check */
>  	X86_TRAP_MC,		/* 18, Machine Check */
>  	X86_TRAP_XF,		/* 19, SIMD Floating-Point Exception */
> +	X86_TRAP_CP = 21,	/* 21 Control Protection Fault */
>  	X86_TRAP_IRET = 32,	/* 32, IRET Exception */
>  };
>  
> diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
> index 87ef69a72c52..8ed406f469e7 100644
> --- a/arch/x86/kernel/idt.c
> +++ b/arch/x86/kernel/idt.c
> @@ -102,6 +102,10 @@ static const __initconst struct idt_data def_idts[] = {
>  #elif defined(CONFIG_X86_32)
>  	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_32),
>  #endif
> +
> +#ifdef CONFIG_X86_64
> +	INTG(X86_TRAP_CP,		control_protection),
> +#endif
>  };
>  
>  /*
> diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> index 9ccbf0576cd0..c572a3de1037 100644
> --- a/arch/x86/kernel/signal_compat.c
> +++ b/arch/x86/kernel/signal_compat.c
> @@ -27,7 +27,7 @@ static inline void signal_compat_build_tests(void)
>  	 */
>  	BUILD_BUG_ON(NSIGILL  != 11);
>  	BUILD_BUG_ON(NSIGFPE  != 15);
> -	BUILD_BUG_ON(NSIGSEGV != 7);
> +	BUILD_BUG_ON(NSIGSEGV != 8);
>  	BUILD_BUG_ON(NSIGBUS  != 5);
>  	BUILD_BUG_ON(NSIGTRAP != 5);
>  	BUILD_BUG_ON(NSIGCHLD != 6);
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 05da6b5b167b..99c83ee522ed 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -570,6 +570,65 @@ do_general_protection(struct pt_regs *regs, long error_code)
>  }
>  NOKPROBE_SYMBOL(do_general_protection);
>  
> +static const char * const control_protection_err[] = {
> +	"unknown",
> +	"near-ret",
> +	"far-ret/iret",
> +	"endbranch",
> +	"rstorssp",
> +	"setssbsy",
> +};
> +
> +/*
> + * When a control protection exception occurs, send a signal
> + * to the responsible application.  Currently, control
> + * protection is only enabled for the user mode.  This
> + * exception should not come from the kernel mode.
> + */
> +dotraplinkage void
> +do_control_protection(struct pt_regs *regs, long error_code)
> +{
> +	struct task_struct *tsk;
> +
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +	if (notify_die(DIE_TRAP, "control protection fault", regs,
> +		       error_code, X86_TRAP_CP, SIGSEGV) == NOTIFY_STOP)
> +		return;
> +	cond_local_irq_enable(regs);
> +
> +	if (!user_mode(regs))
> +		die("kernel control protection fault", regs, error_code);
> +
> +	if (!static_cpu_has(X86_FEATURE_SHSTK) &&
> +	    !static_cpu_has(X86_FEATURE_IBT))
> +		WARN_ONCE(1, "CET is disabled but got control protection fault\n");
> +
> +	tsk = current;
> +	tsk->thread.error_code = error_code;
> +	tsk->thread.trap_nr = X86_TRAP_CP;
> +
> +	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> +	    printk_ratelimit()) {
> +		unsigned int max_err;
> +		unsigned long ssp;
> +
> +		max_err = ARRAY_SIZE(control_protection_err) - 1;
> +		if ((error_code < 0) || (error_code > max_err))
> +			error_code = 0;
> +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
> +		pr_info("%s[%d] control protection ip:%lx sp:%lx ssp:%lx error:%lx(%s)",
> +			tsk->comm, task_pid_nr(tsk),
> +			regs->ip, regs->sp, ssp, error_code,
> +			control_protection_err[error_code]);
> +		print_vma_addr(KERN_CONT " in ", regs->ip);
> +		pr_cont("\n");
> +	}
> +
> +	force_sig_fault(SIGSEGV, SEGV_CPERR,
> +			(void __user *)uprobe_get_trap_addr(regs));
> +}
> +NOKPROBE_SYMBOL(do_control_protection);
> +
>  dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
>  {
>  #ifdef CONFIG_DYNAMIC_FTRACE
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index cb3d6c267181..693071dbe641 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -229,7 +229,8 @@ typedef struct siginfo {
>  #define SEGV_ACCADI	5	/* ADI not enabled for mapped object */
>  #define SEGV_ADIDERR	6	/* Disrupting MCD error */
>  #define SEGV_ADIPERR	7	/* Precise MCD exception */
> -#define NSIGSEGV	7
> +#define SEGV_CPERR	8
> +#define NSIGSEGV	8
>  
>  /*
>   * SIGBUS si_codes
> -- 
> 2.21.0
> 

-- 
Kees Cook