Re: [PATCH v15 04/26] x86/cet: Add control-protection fault handler

Borislav Petkov
2020-11-26T18:49:42Z

On Tue, Nov 10, 2020 at 08:21:49AM -0800, Yu-cheng Yu wrote:
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index e19df6cde35d..6c21c1e92605 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -598,6 +598,65 @@ 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 the user mode. This
> + * exception should not come from the kernel mode.
> + */

Make that 80 cols wide.

> +DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
> +{
> + struct task_struct *tsk;
> +
> + if (notify_die(DIE_TRAP, "control protection fault", regs,
> + error_code, X86_TRAP_CP, SIGSEGV) == NOTIFY_STOP)
> + return;

What is the intent here, notifiers can prevent the machine from printing
the CP error below?

> + cond_local_irq_enable(regs);
> +
> + if (!user_mode(regs))
> + die("kernel control protection fault", regs, error_code);

Let's write that more explicitly:

die("Unexpected/unsupported control protection fault"...

> +
> + if (!static_cpu_has(X86_FEATURE_SHSTK) &&
> + !static_cpu_has(X86_FEATURE_IBT))

Why static_cpu_has?

> + WARN_ONCE(1, "CET is disabled but got control protection fault\n");

"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) &&
> + 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;

<---- newline here.

> + 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");
> + }

...

--
Regards/Gruss,
Boris.