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.wea...@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'?
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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