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

From: Andrew Cooper
Date: Fri Aug 29 2014 - 10:19:39 EST

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.

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