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

Andy Lutomirski
2015-09-28T22:38:51Z

On Mon, Sep 28, 2015 at 2:54 PM, Chris Metcalf <cmetcalf@ezchip.com> wrote:
> On 09/28/2015 04:51 PM, Andy Lutomirski wrote:
>>
>> On Mon, Sep 28, 2015 at 11:17 AM, Chris Metcalf <cmetcalf@ezchip.com>
>>> @@ -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.

It's already like that in Linus' tree.

FWIW, most of those exception handlers send signals, so it might pay
to do it in notify_die or die instead.

>
>>> --- 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.

This should note that it only returns true if context tracking is on.

>>> +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.

For x86, the relevant info might be the actual hw error number
(error_code, which makes it into die) or the signal. If we send a
death signal, then reporting the error number the usual way might make
sense.

--Andy