Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in save_stack_trace_tsk()

[Andy Lutomirski]

On Fri, Sep 16, 2016 at 12:47 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Sep 15, 2016 at 02:19:38PM -0500, Josh Poimboeuf wrote:
>> On Thu, Sep 15, 2016 at 11:41:25AM -0700, Andy Lutomirski wrote:
>> > I also wouldn't mind trying to do something to prevent ever dumping
>> > the stack of an actively running task.  It's definitely safe to dump:
>> >
>> >  - current
>> >
>> >  - any task that's stopped via ptrace, etc
>> >
>> >  - any task on the current CPU if running atomically enough that the
>> > task can't migrate (which probably covers the nasty NMI cases, I hope)
>> >
>> > What's *not* safe AFAIK is /proc/PID/stack.  I don't know if we can
>> > somehow fix that short of actually sending an interrupt or NMI to
>> > freeze the task if it's running.  I'm also not sure it's worth
>> > worrying about it.
>>
>> Yeah, I proposed a fix for /proc/PID/stack a while back:
>>
>>   https://lkml.kernel.org/r/cover.1424109806.git.jpoimboe@xxxxxxxxxx
>>
>> My idea was to use task_rq_lock() to lock the runqueue and then check
>> tsk->on_cpu.  I think Peter wasn't too keen on it.
>
> That basically allows a DoS on the scheduler, since a user can run tasks
> on every cpu (through sys_sched_setaffinity()). Then doing while (1) cat
> /proc/$PID/stack would saturate the rq->lock on every CPU.
>
> The more tasks the merrier.
>
>

Is this worse than it would be if this code used preempt_disable()
(which I think it did until very recently)?

-- 
Andy Lutomirski
AMA Capital Management, LLC