Re: kaslr relocation incompitable with kernel loaded high

[Kees Cook]

On Mon, Apr 21, 2014 at 10:56 AM, Yinghai Lu <yinghai@xxxxxxxxxx> wrote:
> On Mon, Apr 21, 2014 at 3:52 AM, WANG Chao <chaowang@xxxxxxxxxx> wrote:
>> Hi, Kees
>>
>> When I'm testing kaslr with kdump, I find that when 2nd kernel is loaded
>> high, it doesn't boot.
>>
>> I reserved 128M memory at high with kernel cmdline
>> "crashkernel=128M,high crashkernel=0,low", and for which I got:
>>
>> [    0.000000] Reserving 128MB of memory at 6896MB for crashkernel (System RAM: 6013MB)
>>
>> Then I load kdump kernel into the reserved memory region, using a local
>> modified kexec-tools which is passing e820 in boot_params.
>>
>> The e820 map of system RAM passed to 2nd kernel:
>>
>> E820 memmap (of RAM):
>> 0000000000001000-000000000009e3ff (1)
>> 00000001af000000-00000001b6f5dfff (1)
>> 00000001b6fff400-00000001b6ffffff (1)
>>
>> In which, 2nd kernel is loaded at 0x1b5000000.
>>
>> After triggerred a system crash, 2nd kernel doesn't boot even with
>> "nokaslr" cmdline:
>>
>> # echo c > /proc/sysrq-trigger
>> [..]
>>
>> I'm in purgatory
>> early console in decompress_kernel
>> KASLR disabled...
>>
>> Decompressing Linux... Parsing ELF... Performing relocations...
>>
>> 32-bit relocation outside of kernel!
>
> Interesting, when kernel get at "early console in decompress_kernel"
> kernel already in 64 bit...
>
> what does it mean "32-bit relocation outside of kernel" ?
>
> why 32-bit is involved ?

The 64-bit kernel has both 64 and 32 bit relocations (there are two
tables at the end of the kernel image). The error means that the
resulting relocation is believed to be outside the kernel image:

http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x86/boot/compressed/misc.c#n283

Which means there is likely something wrong with this calculation in
your situation:

/*
 * Calculate the delta between where vmlinux was linked to load
 * and where it was actually loaded.
 */
delta = min_addr - LOAD_PHYSICAL_ADDR;


-Kees

-- 
Kees Cook
Chrome OS Security
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