linux-kernel - Re: perf_fuzzer compiled for x32 causes reboot

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Date:	Mon, 24 Feb 2014 18:40:51 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: perf_fuzzer compiled for x32 causes reboot

On Mon, Feb 24, 2014 at 12:32:39PM -0500, Vince Weaver wrote:
> I do note that 
> 	perf_callchain_user();
> 
> Does
> 	fp = (void __user *)regs->bp;
> 	
> 	...
> 
> 	bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
> 
> 
> And in my particular executable RBP has nothing to do with a frame 
> pointer, but is instead being used as a general purpose register.
> 
> Am I missing something here?  Though in that case I'm not sure why this 
> wouldn't be easier to trigger.

Ah, in case the frame doesn't actually exist we would expect to fault
and get the fixup treatment, returning a short copy (the return value
being bytes _NOT_ copied).

When that happens;

  if (bytes != 0)
  	break;

At which point we'll terminate the stack frame iteration.

This is where we rely on being able to take a fault from NMI context,
the fault iret will re-enable NMIs, necessitating all the magic Steve
did.
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