Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

[Kees Cook]

On Wed, Oct 9, 2013 at 3:04 AM, HATAYAMA Daisuke
<d.hatayama@xxxxxxxxxxxxxx> wrote:
> (2013/10/08 22:38), Dave Anderson wrote:
>>
>>
>>
>> ----- Original Message -----
>>>
>>> (2013/10/07 22:21), Dave Anderson wrote:
>>>
>>>> ----- Original Message -----
>>>>>
>>>>> (2013/10/03 22:47), Dave Anderson wrote:
>>>
>>>
>>>>>> ----- Original Message -----
>>>>>>>
>>>>>>> (2013/10/02 18:13), HATAYAMA Daisuke wrote:
>>>>>>>>
>>>>>>>> (2013/10/02 16:48), Kees Cook wrote:
>>>
>>>
>>>>>
>>>>> Thanks for detailed explanation. So, there's already a feature in crash
>>>>> utility
>>>>> to address relocation!, though it's better for me to try them to check
>>>>> if
>>>>> it's
>>>>> really applicable to this feature. My concern is whether --reloc works
>>>>> well
>>>>> on x86_64 too, because relocation has never done on x86_64 ever, right?
>>>>
>>>>
>>>> Correct.
>>>>
>>>>> Another concern is that in case of relocation, users need to additional
>>>>> information
>>>>> regarding runtime symbol information to crash utility. I want to avoid
>>>>> additional
>>>>> process, automation is preferable if possible.
>>>>
>>>>
>>>> Right.  As I mentioned in the case of 32-bit x86 dumpfiles, there is no
>>>> automation
>>>> available when CONFIG_PHYSICAL_START is larger than
>>>> CONFIG_PHYSICAL_ALIGN.  The user
>>>> either has to be aware of their values in order to calculate the --reloc
>>>> argument,
>>>> or has to capture a copy of the /proc/kallsyms file on the crashed
>>>> system.  Typically
>>>> users/distros using kdump changed their x86 configurations to avoid
>>>> having to deal
>>>> with that.
>>>>
>>>
>>> Sorry, I don't understand why relocation size cannot be calculated when
>>> CONFIG_PHYSICALSTART > CONFIG_PHYSICAL_ALIGN. Could you explain that?
>>
>>
>> I just meant that when CONFIG_PHYSICAL_START > CONFIG_PHYSICAL_ALIGN, the
>> 32-bit x86 kernel
>> gets relocated (like the secondary kdump kernel), but that information is
>> not readily available
>> from the vmlinux/vmcore pair.
>>
>
> My understanding on CONFIG_PHYSICAL_ALIGN was that starting address of
> kernel text area
> is always rounded up to CONFIG_PHYSICAL_ALIGN, only. Your explanation would
> be part I don't
> understand well. I'll reconsider it locally...
>
>
>>>
>>>>> I guess it's enough if there's runtime symbol addresses because we can
>>>>> get relocated
>>>>> offset value by comparing it with the compile-time symbol address
>>>>> contained in
>>>>> a given debuginfo file. Candidates for such symbols are the ones
>>>>> contained in
>>>>> VMCOREINFO note containing some symbol values for makedumpfile to refer
>>>>> to mm-related
>>>>> objects in kernel, which is always contained in vmcore generated by
>>>>> current kdump and
>>>>> also vmcores converted by makedumpfile from it. How about this idea?
>>>>
>>>>
>>>> But how would that differ from using an incorrect (non-matching) vmlinux
>>>> file?
>>>>
>>>
>>> It seems to me almost similar to what crash currently does even if we do
>>> relocation check.
>>> The current check crash currently does is trial-and-error since there's
>>> no information
>>> indicating given vmcore and vmlinuxcertainly match well.
>>>
>>> For example, the process I imagine is:
>>>
>>>     1) try to match vmcore and vmlinux with no relocation. If fails, go
>>> to 2).
>>>     2) try to match vmcore and vmlinux with relocation.
>>>
>>> The two steps include symbol table initialization so it might actually be
>>> difficult to
>>> resume back from 2) to 1).
>>>
>>> Also, if gap due to phys_base and gap due to relocation can happen at the
>>> same time,
>>> calculating two values automatically might be futher complicated. So, it
>>> would be better
>>> to add relocation value in VMCOREINFO. Then, what crash utility sholud do
>>> becomes very simple.
>>
>>
>> Yes please...
>>
>> And while you're at it, the kernel's
>>
>>    VMCOREINFO_SYMBOL(phys_base);
>>
>> is pretty much useless, at least w/respect to ELF vmcores, since we need
>> to know its
>> value in order to translate the address.  And I don't think that
>> makedumpfile uses
>> it when it calculates the phys_base that it stores in compressed kdump
>> headers.  Why
>> not put its value instead of its address?
>>
>
> Yes, I've also noticed this fact. Anyway, I'll post a patch to improvement
> this phys_base
> and a patch to export relocation information in VMCOREINFO.

Cool; can you keep me CCed on those patches? I'm interested in seeing
and testing it too.

Thanks!

-Kees

-- 
Kees Cook
Chrome OS Security
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