Re: [resend Patch v3 1/2] kaslr: check if kernel location is changed

["H. Peter Anvin" <hpa@zytor.com>]

On 10/01/2014 06:52 AM, Vivek Goyal wrote:
> 
> Hi Peter,
> 
> I think there is some confusion. I will try to clarify.
> 
> If we have 32bit signed overflow, we will not have a functional kernel.
> And that's the message we get when we try to kexec with
> CONFIG_RANDOMIZE_BASE=y.
> 

And how does that happen?

> **********************************************************************
> [  340.709078] kexec: Starting new kernel
> early console in decompress_kernel
> KASLR disabled by default...
> 
> Decompressing Linux... Parsing ELF...
> 
> Performing relocations...
> 32-bit relocation outside of kernel!
> 
> 
>  -- System halted
> *****************************************************************
> 
> We realized that kexec tries to load kernel at higher physical addresses
> and that can lead to problmes.
> 
> Currently for x86_64, handle_relocations() will perform relocations if
> kernel is not loaded at LOAD_PHYSICAL_ADDR. I think this does not work for
> all the cases and kerenl can not be loaded anywhere in the physical address
> space. And that's why we run into issues with kexec.
> 
> My understanding is that we introduce handle_relcoations() i386 style
> because of RANDOMIZE_BASE. If that's the case, one possible solution
> is that perform relocations only if ranodmize base logic has chosen
> a different load location for kernel than where boot loader loaded
> it. Otherwise don't do anything.
> 
> In case of kexec/kdump, we will pass "nokaslr" to second kernel forcing
> it to do nothing and let the kernel run where it was loaded by bootloader.
> And in that case handle_relocations() should not do any relocations and
> that should allow kernel to be loaded anywhere in physical memory on
> x86_64.
> 

Sorry... this makes no sense.

For x86-64, there is no direct connection between the physical and
virtual address spaces that the kernel runs in...

	-hpa