Re: [RFC PATCH v4 06/27] x86/cet: Control protection exception handler

Yu-cheng Yu
2018-10-03T16:21:51Z

On Wed, 2018-10-03 at 12:39 +0200, Eugene Syromiatnikov wrote:
> On Fri, Sep 21, 2018 at 08:03:30AM -0700, Yu-cheng Yu wrote:
> > +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(KERN_CONT " in ", regs->ip);
> > + pr_cont("\n");
> > + }
> > +
> > + force_sig_info(SIGSEGV, SEND_SIG_PRIV, tsk);
>
> That way, no information is provided to userspace (both application and
> debugger), which is rather unfortunate. It would be nice if a new SEGV_*
> code was added at least, and CET error (with error code constant provided
> in UAPI) is passed via si_errno. (Having ip/sp/*ssp would be even
> better, but I'm not exactly sure about ramifications of providing this
> kind of information to user space).

Ok, I will add that.

Yu-cheng