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

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Message-ID: <alpine.DEB.2.10.1402241230440.20170@vincent-weaver-1.um.maine.edu>
Date:	Mon, 24 Feb 2014 12:32:39 -0500 (EST)
From:	Vince Weaver <vincent.weaver@...ne.edu>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Vince Weaver <vincent.weaver@...ne.edu>,
	"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, 24 Feb 2014, Peter Zijlstra wrote:

> On Mon, Feb 24, 2014 at 12:10:44PM -0500, Vince Weaver wrote:
> > On Mon, 24 Feb 2014, H. Peter Anvin wrote:
> > 
> > > On February 24, 2014 8:34:30 AM PST, Vince Weaver <vincent.weaver@...ne.edu> wrote:
> > > >On Mon, 24 Feb 2014, Vince Weaver wrote:
> > > >
> > > >> Just touching the mmap page with a write of a single byte (it doesn't
> > > >
> > > >> matter where) is enough to trigger the bug.
> > > >
> > > >OK, investigating this more.
> > > >
> > > >perf_fuzzer-2971  [000]   154.944114: page_fault_user:      
> > > >address=0xf7729000 ip=0x41efab error_code=0x6
> > > >perf_fuzzer-2971  [000]   154.944118: function:             
> > > >ip=0xffffffff810d40e7 parent_ip=0xffffffff810d0840
> > > >perf_fuzzer-2971  [000]   154.944119: function:             
> > > >ip=0xffffffff812a91a5 parent_ip=0xffffffff81013ff5
> > > >perf_fuzzer-2971  [000]   154.944120: function:             
> > > >ip=0xffffffff8153837c parent_ip=0xffffffff81535432
> > > >perf_fuzzer-2971  [000]   154.944121: page_fault_kernel:    
> > > >address=0x22e0 ip=0xffffffff812a7d5c error_code=0x0
> > 
> > > Ok, so the obvious question is what is at that kernel address?
> > > 
> > 
> > It's in copy_user_generic_string()
> > 	rep movsq %ds:(%rsi),%es:(%rdi)
> > 
> > And looking at the ftrace:
> >      perf_fuzzer-2979  [000]   161.475920: page_fault_user:      address=__per_cpu_end ip=__per_cpu_end error_code=0x6
> >      perf_fuzzer-2979  [000]   161.475922: function:             perf_callchain
> >      perf_fuzzer-2979  [000]   161.475922: function:             copy_from_user_nmi
> >      perf_fuzzer-2979  [000]   161.475923: function:             trace_do_page_fault
> >      perf_fuzzer-2979  [000]   161.475924: page_fault_kernel:    address=irq_stack_union ip=copy_user_generic_string error_code=0x0
> > 
> > What is likely happening is the user page fault is triggering 
> > code to do a "perf_callchain" dump, which is calling copy_from_user_nmi()
> > which calls copy_user_generic_string() which is somehow getting the user
> > RBP in the RDI register somehow?
> 
> So that code very much relies on the 'recursive' NMI/iret magic from
> Steve, patch 3f3c8b8c4b2a3 (and assorted fixes later).
> 
> If CR2 is getting corrupted; 7fbb98c5cb075 seems relevant.
> 
> Peter, does x32 have a slightly different ABI/calling convention that
> would make any of these patches just slightly 'off'?

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.

Vince
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