From: Vince Weaver <vincent.weaver@maine.edu>
Date: Tue, 9 Jan 2018 10:56:52 -0500 (EST)
To: Peter Zijlstra <peterz@infradead.org>
cc: Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: perf: perf_fuzzer quickly locks up on 4.15-rc7
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On Tue, 9 Jan 2018, Peter Zijlstra wrote:

> So CONFIG_PAGE_TABLE_ISOLATION=y and booting with "pti=off" makes it
> 'work', right?

yes.  Previously I was changing CONFIG_PAGE_TABLE_ISOLATION and 
recompiling, but just now I booted with it set to yes and pti=off and the 
fuzzer has been running fine for a half hour (usually it crashes in under 
5 minutes).

I did see these in the logs which I don't think I've seen before.

WARNING: stack recursion on stack type 2
WARNING: can't dereference iret registers at 000000000783fea8 for ip paranoid_entry+0x2e/0x90
WARNING: can't dereference registers at 00000000f0698d17 for ip paranoid_entry+0x4c/0x90
WARNING: stack going in the wrong direction? ip=native_sched_clock+0x9/0x90

> The below is always my first try to get something out of the machine,
> after that its printk() stuffing code to see how far we get..
> 
> In particular I'd start instrumenting the NMI entry_64.S code, because
> that's really the biggest difference between PTI and !PTI :/ all rather
> bothersome I'm afraid.

I'll try that next.  Also getting a few other machines up and into a 
state that I can start fuzzing on them. 

(extra challenge, the lab my machines is in possibly has a leak in the 
roof, and they're calling for an inch of rain on top of 3 feet of 
existing snow so I might have to shut everything down and relocate on 
short notice).

Vince