[kernel-hardening] Re: [PATCH v5 04/21] x86, boot: Move compressed kernel to end of decompression buffer

[Ingo Molnar <mingo@kernel.org>]

* Kees Cook <keescook@chromium.org> wrote:

> When INIT_SIZE is bigger than VO_INIT_SIZE (uncommon but possible),
> the copied ZO occupies the memory from extract_offset to the end of
> decompression buffer. It overlaps with the soon-to-be-uncompressed kernel
> like this:
> 
>                             |-----compressed kernel image------|
>                             V                                  V
> 0                       extract_offset                      +INIT_SIZE
> |-----------|---------------|-------------------------|--------|
>             |               |                         |        |
>           VO__text      startup_32 of ZO          VO__end    ZO__end
>             ^                                         ^
>             |-------uncompressed kernel image---------|
> 
> When INIT_SIZE is equal to VO_INIT_SIZE (likely) there's still space
> left from end of ZO to the end of decompressing buffer, like below.
> 
>                             |-compressed kernel image-|
>                             V                         V
> 0                       extract_offset                      +INIT_SIZE
> |-----------|---------------|-------------------------|--------|
>             |               |                         |        |
>           VO__text      startup_32 of ZO          ZO__end    VO__end
>             ^                                                  ^
>             |------------uncompressed kernel image-------------|
> 
> To simplify calculations and avoid special cases, it is cleaner to
> always place the compressed kernel image in memory so that ZO__end
> is at the end of the decompression buffer, instead of placing that
> start extract_offset as is currently done.

Btw., it would be nice to also put such a visualization (of the current layout of 
these values) into the code itself.

Thanks,

	Ingo