Message-ID: <52521566.20402@jp.fujitsu.com>
Date:	Mon, 07 Oct 2013 10:59:02 +0900
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	Dave Anderson <anderson@...hat.com>
CC:	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	kernel-hardening@...ts.openwall.com,
	Aaron Durbin <adurbin@...gle.com>,
	Eric Northup <digitaleric@...gle.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...gle.com>,
	Mathias Krause <minipli@...glemail.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Discussion list for crash utility usage, maintenance and development" 
	<crash-utility@...hat.com>
Subject: Re: [PATCH 6/7] x86, kaslr: report kernel offset on panic

(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:
>> <cut>
>>>>>> +
>>>>>> +     return 0;
>>>>>> +}
>>>>>> +
>>>>>> +/*
>>>>>>      * Determine if we were loaded by an EFI loader.  If so, then we have also been
>>>>>>      * passed the efi memmap, systab, etc., so we should use these data structures
>>>>>>      * for initialization.  Note, the efi init code path is determined by the
>>>>>> @@ -1242,3 +1256,15 @@ void __init i386_reserve_resources(void)
>>>>>>     }
>>>>>>
>>>>>>     #endif /* CONFIG_X86_32 */
>>>>>> +
>>>>>> +static struct notifier_block kernel_offset_notifier = {
>>>>>> +     .notifier_call = dump_kernel_offset
>>>>>> +};
>>>>>> +
>>>>>> +static int __init register_kernel_offset_dumper(void)
>>>>>> +{
>>>>>> +     atomic_notifier_chain_register(&panic_notifier_list,
>>>>>> +                                     &kernel_offset_notifier);
>>>>>> +     return 0;
>>>>>> +}
>>>>>> +__initcall(register_kernel_offset_dumper);
>>>>>>
>>>>>
>>>>> Panic notifier is not executed if kdump is enabled. Maybe, Chrome OS doesn't use
>>>>> kdump? Anyway, kdump related tools now calculate phys_base from memory map
>>>>> information passed as ELF PT_LOAD entries like below.
>>>>
>>>> Correct, we are not currently using kdump.
>>>>
>>>>> $ LANG=C readelf -l vmcore-rhel6up4
>>>>>
>>>>> Elf file type is CORE (Core file)
>>>>> Entry point 0x0
>>>>> There are 5 program headers, starting at offset 64
>>>>>
>>>>> Program Headers:
>>>>>     Type           Offset             VirtAddr           PhysAddr
>>>>>                    FileSiz            MemSiz              Flags  Align
>>>>>     NOTE           0x0000000000000158 0x0000000000000000  0x0000000000000000
>>>>>                    0x0000000000000b08 0x0000000000000b08         0
>>>>>     LOAD           0x0000000000000c60 0xffffffff81000000  0x0000000001000000
>>>>>                    0x000000000103b000 0x000000000103b000  RWE    0
>>>>>     LOAD           0x000000000103bc60 0xffff880000001000  0x0000000000001000
>>>>>                    0x000000000009cc00 0x000000000009cc00  RWE    0
>>>>>     LOAD           0x00000000010d8860 0xffff880000100000  0x0000000000100000
>>>>>                    0x0000000002f00000 0x0000000002f00000  RWE    0
>>>>>     LOAD           0x0000000003fd8860 0xffff880013000000  0x0000000013000000
>>>>>                    0x000000002cffd000 0x000000002cffd000  RWE    0
>>>>>
>>>>> Each PT_LOAD entry is assigned to virtual and physical address. In this case,
>>>>> 1st PT_LOAD entry belongs to kernel text mapping region, from which we can
>>>>> calculate phys_base value.
>>>>
>>>> It seems like all the information you need would still be available?
>>>> The virtual address is there, so it should be trivial to see the
>>>> offset, IIUC.
>>>>
>>>
>>> Partially yes. I think OK to analyze crash dump by crash utility, a gdb-based
>>> symbolic debugger for kernel, since phys_base absorbs kernel offset caused by
>>> relocation and phys_base is available in the way I explained above.
>>>
>>> However, the gained phys_base is not correct one, exactly phys_base + offset_by_relocation.
>>> When analyzing crash dump by crash utility, we use debug information generated
>>> during kernel build, which we install as kernel-debuginfo on RHEL for example.
>>> Symbols in debuginfo have statically assigned addresses at build so we see
>>> the statically assigned addresses during debugging and we see
>>> phys_base + offset_by_relocation as phys_base. This would be problematic
>>> if failure on crash dump is relevant to the relocated addresses, though I don't
>>> immediately come up with crash senario where relocated symbol is defitely necessary.
>>>
>>> Still we can get relocated addresses if kallsyms is enabled on the kernel,
>>> but kallsyms and relocatable kernels are authogonal. I don't think it natural
>>> to rely on kallsyms. It seems natural to export relocation information newly
>>> as debugging information.
>>>
>>
>> I was confused yesterday. As I said above, kdump related tools now don't support
>> relocation on x86_64, phys_base only. kdump related tools think of present kernel
>> offset as phys_base. Then, they reflect kernel offset caused by relocation in
>> physical addresses only, not in virtual addresses. This obviously affects the
>> tools.
>>
>> BTW, relocation looks more sophisticated than phys_base one. Is it possible to
>> switch from phys_base one to relocation on x86_64? On x86, relocation is used so
>> I guess x86_64 can work in the same way. Is there something missing?
>> Is there what phys_base can but relocation cannot on x86_64?
>>
>> And, Dave, is there feature for crash utility to treat relocation now?
>
> Well sort of, there are couple guessing-game kludges that can be used.
>
> For 32-bit x86 systems configured with a CONFIG_PHYSICAL_START value
> that is larger than its CONFIG_PHYSICAL_ALIGN value, such that the
> vmlinux symbol values do not match their relocated virtual address
> values, there are two options for analyzing dumpfiles:
>
> (1) there is a "--reloc size" command line option, presuming that
>      you know what it is.
> (2) take a snapshot of the /proc/kallsyms file from the crashing
>      system into a file, and put it on the command line, similar
>      to putting a System.map file on the command line in order to
>      override the symbol values in the vmlinux file.
>
> In those cases, we have to alter all of the symbols seen in the
> vmlinux file, and go into a backdoor into the embedded gdb module
> to patch/modify the symbol values.
>
> On live x86 systems, the two options above are not necessary if
> /proc/kallsyms exists, because its contents can be checked against
> the vmlinux file symbol values, and the relocation calculated.
>
> For x86_64, the --reloc argument has never been needed.  But if
> for whatever reason the "phys_base" value cannot be determined,
> it can be forced with the "--machdep phys_base=addr" option,
> again presuming you know what it is.
>

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?

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.

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?

# I added CC to crash utility mailing list

-- 
Thanks.
HATAYAMA, Daisuke

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