Re: [PATCH v3 07/19] x86, kaslr: Get correct max_addr for relocs pointer

[Kees Cook]

On Fri, Mar 4, 2016 at 8:25 AM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> From: Yinghai Lu <yinghai@xxxxxxxxxx>
>
> There is boundary checking for pointer in kaslr relocation handling.
>
> Current code is using output_len, and that is VO (vmlinux after objcopy)
> file size plus vmlinux.relocs file size.
>
> That is not right, as we should use loaded address for running.

I think this needs clarification. If I'm understanding correctly, the
max_addr check should stop at the end of VO, and not include bss, brk,
etc. which follows. In which case, this change is correct: the bounds
checking needed to be tightened.

-Kees

>
> At that time parse_elf already move the sections according to ELF headers.
>
> The valid range should be VO [_text, __bss_start) loaded physical addresses.
>
> In the patch, add export for __bss_start to voffset.h and use it to get
> max_addr.
>
> Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> ---
> v2->v3:
>     Tune the patch log.
>
>  arch/x86/boot/compressed/Makefile | 2 +-
>  arch/x86/boot/compressed/misc.c   | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index fef80fa..2e7c0ce 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -41,7 +41,7 @@ LDFLAGS_vmlinux := -T
>  hostprogs-y    := mkpiggy
>  HOST_EXTRACFLAGS += -I$(srctree)/tools/include
>
> -sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(_text\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
> +sed-voffset := -e 's/^\([0-9a-fA-F]*\) [ABCDGRSTVW] \(_text\|__bss_start\|_end\)$$/\#define VO_\2 _AC(0x\1,UL)/p'
>
>  quiet_cmd_voffset = VOFFSET $@
>        cmd_voffset = $(NM) $< | sed -n $(sed-voffset) > $@
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 069120e..dd7ed8a 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -259,7 +259,7 @@ static void handle_relocations(void *output, unsigned long output_len)
>         int *reloc;
>         unsigned long delta, map, ptr;
>         unsigned long min_addr = (unsigned long)output;
> -       unsigned long max_addr = min_addr + output_len;
> +       unsigned long max_addr = min_addr + (VO___bss_start - VO__text);
>
>         /*
>          * Calculate the delta between where vmlinux was linked to load
> --
> 2.5.0
>



-- 
Kees Cook
Chrome OS & Brillo Security