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

Chris Metcalf
2015-09-28T21:54:52Z

On 09/28/2015 04:51 PM, Andy Lutomirski wrote:
> On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf@ezchip.com> 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.

I could be convinced to do it either way. My initial thinking was that
a security violation was more interesting and more important to
report than a strict-mode task-isolation violation. But see my
comments in response to your email on patch 07/11.

>> @@ -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.

Yes, in which case we'd have to do it the same way we are doing
it for arm64 (see patch 09/11), by calling task_isolation_exception()
explicitly from within the relevant exception handlers. If we start
doing that, it's probably worth wrapping up the logic into a single
inline function to keep the added code short and sweet.

If in fact this might happen in the short term, it might be a good
idea to hook the individual exception handlers in x86 now, and not
hook the exception_enter() mechanism at all.

>> --- 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.

Added:

* Return: if called with state == CONTEXT_USER, the function returns
* true if we were in fact previously in user mode.

>> +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?

Yup, next patch.

>> +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?

I could modify it to take a string argument (and then use it for
the arm64 case at least). For the exception_enter() caller, we actually
don't have the information available to pass down, and it would
be hard to get it.

--
Chris Metcalf, EZChip Semiconductor