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

[Baoquan He]

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.
********************

> >
> > 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