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Date: Mon, 24 Feb 2014 18:25:36 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vince Weaver <vincent.weaver@maine.edu>
Cc: "H. Peter Anvin" <hpa@zytor.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>
Subject: Re: perf_fuzzer compiled for x32 causes reboot
Message-ID: <20140224172536.GD9987@twins.programming.kicks-ass.net>
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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@maine.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'?