Re: [PATCH v3 1/7] x86, kaslr: Use init_size instead of run_size

[Yinghai Lu]

On Mon, Mar 9, 2015 at 5:49 AM, Borislav Petkov <bp@xxxxxxx> wrote:
> I ended up committing this. Anything I've missed?
>
> ---
> From: Yinghai Lu <yinghai@xxxxxxxxxx>
> Date: Sat, 7 Mar 2015 14:07:15 -0800
> Subject: [PATCH] x86/setup: Use init_size instead of run_size
>
> Commit
>
>   e6023367d779 ("x86, kaslr: Prevent .bss from overlaping initrd")
>
> introduced run_size for KASLR to represent the size of kernel proper
> (vmlinux).
>
> However, we should use the actual runtime size (which provides for
> copy/decompress), i.e. init_size, as it includes .bss and .brk.
>
> Why, you ask?
>
> Because init_size is the size needed for safe kernel decompression and
> thus can be higher than run_size in case the decompressor needs a larger
> buffer.
>
> From arch/x86/boot/header.S:
>   #define ZO_INIT_SIZE    (ZO__end - ZO_startup_32 + ZO_z_extract_offset)
>   #define VO_INIT_SIZE    (VO__end - VO__text)
>   #if ZO_INIT_SIZE > VO_INIT_SIZE
>   #define INIT_SIZE ZO_INIT_SIZE
>   #else
>   #define INIT_SIZE VO_INIT_SIZE
>   #endif
>   init_size:              .long INIT_SIZE         # kernel initialization size
>
> The boot loader allocates a buffer of size init_size which it
> reads from the setup header and loads the compressed kernel
> (arch/x86/boot/compressed/vmlinux) in it.
>
> init_size initially comes from the kernel proper's (vmlinux) init size.
> It includes the .bss and .brk area.
>
> When the boot loader hands off to the compressed kernel, the last
> moves itself to z_extract_offset within the buffer to make sure that
> the decompressor output does not overwrite input data before it gets
> consumed.
>
> However, z_extract_offset is the size difference
> between the uncompressed and compressed kernel (see
> arch/x86/boot/compressed/mkpiggy.c) and thus represents the additional
> space needed for decompression but it doesn't factor in a bigger
> ZO_INIT_SIZE.

Can you put back:
"
So need to make sure [z_extra_offset, init_size) will fit ZO, that means
init_size need to be adjusted according to ZO size.
That make init_size is always >= run_size.
"

>
> During ASLR buffer searching, we need to make sure the new buffer is big
> enough for decompression. So use init_size instead, and kill run_size
> related code.
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