Re: [PATCH v7 03/11] task_isolation: support PR_TASK_ISOLATION_STRICT mode

Andy Lutomirski
2015-09-28T20:51:49Z

On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf-d5a29ZRxExrQT0dZR+AlfA@public.gmane.org> wrote:
> With task_isolation mode, the task is in principle guaranteed not to
> be interrupted by the kernel, but only if it behaves. In particular,
> if it enters the kernel via system call, page fault, or any of a
> number of other synchronous traps, it may be unexpectedly exposed
> to long latencies. Add a simple flag that puts the process into
> a state where any such kernel entry is fatal; this is defined as
> happening immediately after the SECCOMP test.

Why after seccomp? Seccomp is still an entry, and the code would be
considerably simpler if it were before seccomp.

> @@ -35,8 +36,12 @@ static inline enum ctx_state exception_enter(void)
> return 0;
>
> prev_ctx = this_cpu_read(context_tracking.state);
> - if (prev_ctx != CONTEXT_KERNEL)
> - context_tracking_exit(prev_ctx);
> + if (prev_ctx != CONTEXT_KERNEL) {
> + if (context_tracking_exit(prev_ctx)) {
> + if (task_isolation_strict())
> + task_isolation_exception();
> + }
> + }
>
> return prev_ctx;
> }

x86 does not promise to call this function. In fact, x86 is rather
likely to stop ever calling this function in the reasonably near
future.

> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -144,15 +144,16 @@ NOKPROBE_SYMBOL(context_tracking_user_enter);
> * This call supports re-entrancy. This way it can be called from any exception
> * handler without needing to know if we came from userspace or not.
> */
> -void context_tracking_exit(enum ctx_state state)
> +bool context_tracking_exit(enum ctx_state state)

This needs clear documentation of what the return value means.

> +static void kill_task_isolation_strict_task(void)
> +{
> + /* RCU should have been enabled prior to this point. */
> + RCU_LOCKDEP_WARN(!rcu_is_watching(), "kernel entry without RCU");
> +
> + dump_stack();
> + current->task_isolation_flags &= ~PR_TASK_ISOLATION_ENABLE;
> + send_sig(SIGKILL, current, 1);
> +}

Wasn't this supposed to be configurable? Or is that something that
happens later on in the series?

> +
> +/*
> + * This routine is called from syscall entry (with the syscall number
> + * passed in) if the _STRICT flag is set.
> + */
> +void task_isolation_syscall(int syscall)
> +{
> + /* Ignore prctl() syscalls or any task exit. */
> + switch (syscall) {
> + case __NR_prctl:
> + case __NR_exit:
> + case __NR_exit_group:
> + return;
> + }
> +
> + pr_warn("%s/%d: task_isolation strict mode violated by syscall %d\n",
> + current->comm, current->pid, syscall);
> + kill_task_isolation_strict_task();
> +}

Ick. I guess it works, but this is still quite ugly IMO.

> +void task_isolation_exception(void)
> +{
> + pr_warn("%s/%d: task_isolation strict mode violated by exception\n",
> + current->comm, current->pid);
> + kill_task_isolation_strict_task();
> +}

Should this say what exception?

--Andy