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

[Kees Cook]

On Mon, Mar 7, 2016 at 9:13 PM, Baoquan He <bhe@xxxxxxxxxx> wrote:
> On 03/07/16 at 03:16pm, Kees Cook wrote:
>> 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.
>
> Yes, exactly as you means.
>
>>
>>
>> >
>> > 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.
>
> I think it is expressed here. Maybe we can stress it in this patch log.
> After all it's the main idea of this patch.
>
> How about rewriting patch log like this:
>
> ********************
> There is boundary checking for pointer in kaslr relocation handling.
> However current code is using output_len that is VO (vmlinux after objcopy)
> file size plus vmlinux.relocs file size to make the valid boundary.
>
> That is not right since the max_addr check should stop at the end of VO
> and excludes bss, brk etc, which follows. Means the valid range should
> be VO [_text, __bss_start] in loaded physical address space.
>
> In this patch, add export for __bss_start to voffset.h and use it to get
> max_addr.
> ********************

I would say:

Relocation handling performs bounds checking on the relocated
addresses. The existing code uses output_len (VO size plus relocs
size) as the max address. This is not right since the max_addr check
should stop at the end of VO and exclude bss, brk, etc, which follows.
The valid range should be VO [_text, __bss_start] in the loaded
physical address space.

In this patch, add export for __bss_start to voffset.h and use it to
get the correct max_addr.


Thanks!

-Kees

>
>> >
>> > 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



-- 
Kees Cook
Chrome OS & Brillo Security