Re: [Xen-devel] [PATCH] Solved the Xen PV/KASLR riddle

Stefan Bader
2014-08-29T14:32:42Z

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On 29.08.2014 16:19, Andrew Cooper wrote:
> On 29/08/14 09:37, Stefan Bader wrote:
>> On 29.08.2014 00:42, Andrew Cooper wrote:
>>> On 28/08/2014 19:01, Stefan Bader wrote:
>>>>>> So not much further... but then I think I know what I do next. Probably should
>>>>>> have done before. I'll replace the WARN_ON in vmalloc that triggers by a panic
>>>>>> and at least get a crash dump of that situation when it occurs. Then I can dig
>>>>>> in there with crash (really should have thought of that before)...
>>>>> <nods> I dug a bit in the code (arch/x86/xen/mmu.c) but there is nothing there
>>>>> that screams at me, so I fear I will have to wait until you get the crash
>>>>> and get some clues from that.
>>>> Ok, what a journey. So after long hours of painful staring at the code...
>>>> (and btw, if someone could tell me how the heck one can do a mfn_to_pfn
>>>> in crash, I really would appreaciate :-P)
>>> The M2P map lives in the Xen reserved virtual address space in each PV
>>> guest, and forms part of the PV ABI. It is mapped read-only, in the
>>> native width of the guest.
>>>
>>> 32bit PV (PAE) at 0xF5800000
>>> 64bit PV at 0xFFFF800000000000
>>>
>>> This is represented by the MACH2PHYS_VIRT_START symbol from the Xen
>>> public header files. You should be able to blindly construct a pointer
>>> to it (if you have nothing better to hand), as it will be hooked into
>>> the guests pagetables before execution starts. Therefore,
>>> "MACH2PHYS_VIRT_START[(unsigned long)pfn]" ought to do in a pinch.
>> machine_to_phys_mapping is set to that address but its not mapped inside the
>> crash dump. Somehow vtop in crash handles translations. I need to have a look at
>> their code, I guess.
>>
>> Thanks,
>> Stefan
>
> What context is the crash dump? If it is a Xen+dom0 kexec()d to native
> linux, then the m2p should still be accessible given dom0's cr3. If it
> is some state copied off-host then you will need to adjust the copy to
> include that virtual range.

No its a domU dump of a PV guest taken with "xl dump-core" (or actually the
result of on-crash trigger).

>
> ~Andrew
>

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