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SubjectRe: [RFC PATCH v3 06/24] x86/cet: Control protection exception handler
On Thu, Aug 30, 2018 at 4:43 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> A control protection exception is triggered when a control flow transfer
> attempt violated shadow stack or indirect branch tracking constraints.
> For example, the return address for a RET instruction differs from the
> safe copy on the shadow stack; or a JMP instruction arrives at a non-
> ENDBR instruction.
>
> The control protection exception handler works in a similar way as the
> general protection fault handler.

Is there a reason why all the code in this patch isn't #ifdef'ed away
on builds that don't support CET? It looks like the CET handler is
hooked up to the IDT conditionally, but the handler code is always
built?

> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> ---
> arch/x86/entry/entry_64.S | 2 +-
> arch/x86/include/asm/traps.h | 3 ++
> arch/x86/kernel/idt.c | 4 +++
> arch/x86/kernel/traps.c | 58 ++++++++++++++++++++++++++++++++++++
> 4 files changed, 66 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 957dfb693ecc..5f4914e988df 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -1000,7 +1000,7 @@ 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 3de69330e6c5..5196050ff3d5 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);
> @@ -77,6 +78,7 @@ dotraplinkage void do_stack_segment(struct pt_regs *, long);
> dotraplinkage void do_double_fault(struct pt_regs *, long);
> #endif
> dotraplinkage void do_general_protection(struct pt_regs *, long);
> +dotraplinkage void do_control_protection(struct pt_regs *, long);
> dotraplinkage void do_page_fault(struct pt_regs *, unsigned long);
> dotraplinkage void do_spurious_interrupt_bug(struct pt_regs *, long);
> dotraplinkage void do_coprocessor_error(struct pt_regs *, long);
> @@ -142,6 +144,7 @@ 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 01adea278a71..2d02fdd599a2 100644
> --- a/arch/x86/kernel/idt.c
> +++ b/arch/x86/kernel/idt.c
> @@ -104,6 +104,10 @@ static const __initconst struct idt_data def_idts[] = {
> #elif defined(CONFIG_X86_32)
> SYSG(IA32_SYSCALL_VECTOR, entry_INT80_32),
> #endif
> +
> +#ifdef CONFIG_X86_INTEL_CET
> + INTG(X86_TRAP_CP, control_protection),
> +#endif
> };
>
> /*
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index e6db475164ed..21a713b96148 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -578,6 +578,64 @@ do_general_protection(struct pt_regs *regs, long error_code)
> }
> NOKPROBE_SYMBOL(do_general_protection);
>
> +static const char *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;
> +
> + max_err = ARRAY_SIZE(control_protection_err) - 1;
> + if ((error_code < 0) || (error_code > max_err))
> + error_code = 0;
> + pr_info("%s[%d] control protection ip:%lx sp:%lx error:%lx(%s)",
> + tsk->comm, task_pid_nr(tsk),
> + regs->ip, regs->sp, error_code,
> + control_protection_err[error_code]);
> + print_vma_addr(" in ", regs->ip);

Shouldn't this be using KERN_CONT, like other callers of
print_vma_addr(), to get the desired output?

> + pr_cont("\n");
> + }
> +
> + force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);

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