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

From: Stefan Bader
Date: Fri Aug 29 2014 - 04:38:12 EST

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.

> ~Andrew

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