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SubjectRe: [PATCH 6/7] x86, kaslr: report kernel offset on panic
(2013/10/02 16:48), Kees Cook wrote:
> On Tue, Oct 1, 2013 at 5:38 PM, HATAYAMA Daisuke
> <d.hatayama@jp.fujitsu.com> wrote:
>> (2013/10/02 4:37), Kees Cook wrote:
>>> When the system panics, include the kernel offset in the report to assist
>>> in debugging.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> ---
>>> arch/x86/kernel/setup.c | 26 ++++++++++++++++++++++++++
>>> 1 file changed, 26 insertions(+)
>>>
>>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>>> index f0de629..1708862 100644
>>> --- a/arch/x86/kernel/setup.c
>>> +++ b/arch/x86/kernel/setup.c
>>> @@ -824,6 +824,20 @@ static void __init trim_low_memory_range(void)
>>> }
>>>
>>> /*
>>> + * Dump out kernel offset information on panic.
>>> + */
>>> +static int
>>> +dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
>>> +{
>>> + pr_emerg("Kernel Offset: 0x%lx from 0x%lx "
>>> + "(relocation range: 0x%lx-0x%lx)\n",
>>> + (unsigned long)&_text - __START_KERNEL, __START_KERNEL,
>>> + __START_KERNEL_map, MODULES_VADDR-1);
>>
>> Using phys_base seems better.
>
> phys_base is not changed during relocation. For example, if I print
> out phys_base during this pr_emerg call, it remains 0x0, even though
> the random offset was 0xa200000.
>

Thanks, I'm clear. The relocation changes addresses assigned to kernel symbols
themselves and kernel text region is loaded into the newly assigned relocated
address. So, phys_base results in 0.

>>> +
>>> + 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.

>> Therefore, we already have phys_base information even in case of kdump, and
>> as long as using kdump-related tools such as crash utility, we don't need
>> to see ELF PT_LOAD headers as I explain here because they calculate the process
>> I explain here automatically.
>>
>> Another idea is to add phys_base value in VMCOREINFO information that is debugging
>> information for user-land tools to filter crash dump. This is simple string information
>> so you can see the values contained in some crash dump by using strings command to it.
>> For example,
>
> Sure, though not phys_base, but rather the offset? I think this is
> similar to coredumps of PIE binaries end up with, though I haven't
> looked very closely at that in a while.
>
> -Kees
>


--
Thanks.
HATAYAMA, Daisuke



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