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

Kees Cook
2020-02-25T20:06:13Z

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@intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-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