Re: [PATCH v19 06/25] x86/cet: Add control-protection fault handler

[Kees Cook]

On Wed, Feb 03, 2021 at 02:55:28PM -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.
> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> ---
>  arch/x86/include/asm/idtentry.h    |  4 ++
>  arch/x86/kernel/idt.c              |  4 ++
>  arch/x86/kernel/signal_compat.c    |  2 +-
>  arch/x86/kernel/traps.c            | 60 ++++++++++++++++++++++++++++++
>  include/uapi/asm-generic/siginfo.h |  3 +-
>  5 files changed, 71 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
> index f656aabd1545..ff4b3bf634da 100644
> --- a/arch/x86/include/asm/idtentry.h
> +++ b/arch/x86/include/asm/idtentry.h
> @@ -574,6 +574,10 @@ DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_SS,	exc_stack_segment);
>  DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_GP,	exc_general_protection);
>  DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_AC,	exc_alignment_check);
>  
> +#ifdef CONFIG_X86_CET
> +DECLARE_IDTENTRY_ERRORCODE(X86_TRAP_CP, exc_control_protection);
> +#endif
> +
>  /* Raw exception entries which need extra work */
>  DECLARE_IDTENTRY_RAW(X86_TRAP_UD,		exc_invalid_op);
>  DECLARE_IDTENTRY_RAW(X86_TRAP_BP,		exc_int3);
> diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
> index ee1a283f8e96..e8166d9bbb10 100644
> --- a/arch/x86/kernel/idt.c
> +++ b/arch/x86/kernel/idt.c
> @@ -105,6 +105,10 @@ static const __initconst struct idt_data def_idts[] = {
>  #elif defined(CONFIG_X86_32)
>  	SYSG(IA32_SYSCALL_VECTOR,	entry_INT80_32),
>  #endif
> +
> +#ifdef CONFIG_X86_CET
> +	INTG(X86_TRAP_CP,		asm_exc_control_protection),
> +#endif
>  };
>  
>  /*
> diff --git a/arch/x86/kernel/signal_compat.c b/arch/x86/kernel/signal_compat.c
> index a5330ff498f0..dd92490b1e7f 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 != 9);
> +	BUILD_BUG_ON(NSIGSEGV != 10);
>  	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 7f5aec758f0e..f5354c35df32 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -606,6 +606,66 @@ DEFINE_IDTENTRY_ERRORCODE(exc_general_protection)
>  	cond_local_irq_disable(regs);
>  }
>  
> +#ifdef CONFIG_X86_CET
> +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 user mode.
> + * This exception should not come from kernel mode.
> + */
> +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> +{
> +	static DEFINE_RATELIMIT_STATE(rs, DEFAULT_RATELIMIT_INTERVAL,
> +				      DEFAULT_RATELIMIT_BURST);
> +	struct task_struct *tsk;
> +
> +	if (!user_mode(regs)) {
> +		pr_emerg("PANIC: unexpected kernel control protection fault\n");
> +		die("kernel control protection fault", regs, error_code);
> +		panic("Machine halted.");
> +	}
> +
> +	cond_local_irq_enable(regs);
> +
> +	if (!boot_cpu_has(X86_FEATURE_CET))
> +		WARN_ONCE(1, "Control protection fault with CET support disabled\n");
> +
> +	tsk = current;
> +	tsk->thread.error_code = error_code;
> +	tsk->thread.trap_nr = X86_TRAP_CP;
> +
> +	if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> +	    __ratelimit(&rs)) {
> +		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;

Do you want to mask the error_code here before printing its value?

> +
> +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
> +		pr_emerg("%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]);

Instead, you could clamp error_code to ARRAY_SIZE(control_protection_err),
and add another "unknown" to the end of the strings:

	control_protection_err[
		array_index_nospec(error_code,
				   ARRAY_SIZE(control_protection_err))]

Everything else looks good.

> +		print_vma_addr(KERN_CONT " in ", regs->ip);
> +		pr_cont("\n");
> +	}
> +
> +	force_sig_fault(SIGSEGV, SEGV_CPERR,
> +			(void __user *)uprobe_get_trap_addr(regs));
> +	cond_local_irq_disable(regs);
> +}
> +#endif
> +
>  static bool do_int3(struct pt_regs *regs)
>  {
>  	int res;
> diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> index d2597000407a..1c2ea91284a0 100644
> --- a/include/uapi/asm-generic/siginfo.h
> +++ b/include/uapi/asm-generic/siginfo.h
> @@ -231,7 +231,8 @@ 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
> -- 
> 2.21.0
> 

-- 
Kees Cook