Re: [PATCH v21 06/26] x86/cet: Add control-protection fault handler

Borislav Petkov
2021-02-24T16:16:56Z

On Wed, Feb 17, 2021 at 02:27:10PM -0800, Yu-cheng Yu wrote:
> +/*
> + * 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);

Pls move that out of the function - those "static" qualifiers get missed
easily when inside a function.

> + 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;
> +
> + /*
> + * Ratelimit to prevent log spamming.
> + */
> + if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) &&
> + __ratelimit(&rs)) {
> + unsigned long ssp;
> + int err;
> +
> + err = array_index_nospec(error_code, ARRAY_SIZE(control_protection_err));

"err" as an automatic variable is confusing - we use those to denote
whether the function returned an error or not. Call yours "cpf_type" or
so.

> +
> + 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[err]);
> + print_vma_addr(KERN_CONT " in ", regs->ip);
> + pr_cont("\n");
> + }
> +
> + force_sig_fault(SIGSEGV, SEGV_CPERR,
> + (void __user *)uprobe_get_trap_addr(regs));

Why is this calling an uprobes function?

Also, do not break that line even if it is longer than 80.

> + 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

I still don't see the patch adding this to the manpage of sigaction(2).

There's a git repo there: https://www.kernel.org/doc/man-pages/

and I'm pretty sure Michael takes patches.

--
Regards/Gruss,
Boris.