Re: [PATCH v13 (resend) 2/6] x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls

[Ingo Molnar <mingo@kernel.org>]

* Alexander Popov <alex.popov@linux.com> wrote:

> Hello Ingo,
> 
> Thanks for your review! I'll fix the style issues you pointed at.
> 
> Please also see my answers below.
> 
> On 05.07.2018 11:12, Ingo Molnar wrote:
> >> +	  The tradeoff is the performance impact: on a single CPU system kernel
> >> +	  compilation sees a 1% slowdown, other systems and workloads may vary
> >> +	  and you are advised to test this feature on your expected workload
> >> +	  before deploying it.
> > 
> > Is there a way to patch this out runtime? I.e. if a distro enabled it, is there an 
> > easy way to disable much of the overhead without rebooting the kernel?
> 
> Hm. We can't completely disable STACKLEAK in runtime, since STACKLEAK gcc plugin
> performs compile-time instrumentation of the kernel code. So we can only chop
> off a part of functionality, for example, by introducing some variable and
> checking it before every stack erasing (additional performance impact), but the
> kernel will stay uselessly instrumented. It doesn't look reasonable to me.

Or we could use what every other performance critical instrumentation feature uses 
to reduce overhead (ftrace, perf): kernel patching.

> > If so then please make this:
> > 
> > 	if (WARN_ON(boundary - kstack_ptr >= THREAD_SIZE))
> > 		return;
> > 
> > or so, to make it non-fatal and to allow users to report it, should it trigger 
> > against all expectations.
> 
> I've made an experiment. The results:
>  1. BUG_ON() here doesn't freeze the kernel output - I see a full 'PANIC: double
> fault' report;

Only in text mode - very few users are using text mode.

>  2. WARN_ON() here gives absolutely same 'PANIC: double fault' here.

that should only happen if the kernel is otherwise already fatally corrupted, 
right?

Thanks,

	Ingo