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

From: H. Peter Anvin
Date: Tue Sep 30 2014 - 17:22:07 EST

On 09/30/2014 12:08 AM, Baoquan He wrote:
> Function handle_relocations() is used to do the relocations handling
> for i686 and kaslr of x86_64. For 32 bit the relocation handling is
> mandotary to perform. For x86_64 only when kaslr is enabled and a
> random kernel location is chosen successfully the relocation handling
> shound be done. However previous implementation only compared the
> kernel loading address and LOAD_PHYSICAL_ADDR where kernel were
> compiled to run at. This would casue system to be exceptional in
> few conditions like when delta between load address and compiled
> address is bigger than what 32bit signed relocations can handle.
> Also there will be limitations that delta can't be too big otherwise
> kernel text virtual addresses will overflow in module address space.
> So in this patch check if kernel location is changed after
> choose_kernel_location() when x86_64. If and only if in x86_64
> and kernel location is changed, we say a kaslr random kernel
> location is chosen, then the relocation handling is needed.
> Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> Acked-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Tested-by: Thomas D. <whissi@xxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Could you clarify under what conditions we may end up with 32-bit signed
overflow, and yet have a functional kernel?


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