Re: [PATCH 2/4] kaslr: check if the random addr is available

Baoquan He
2014-09-05T22:17:19Z

On 09/05/14 at 10:16am, Kees Cook wrote:
> On Fri, Sep 5, 2014 at 7:08 AM, Baoquan He <bhe@redhat.com> wrote:
> > diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> > index 7780a5b..d2a0eaa 100644
> > --- a/arch/x86/boot/compressed/misc.c
> > +++ b/arch/x86/boot/compressed/misc.c
> > @@ -250,6 +250,11 @@ static void handle_relocations(void *output, unsigned long output_len)
> > }
> > #endif
> >
> > + if (max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET) {
> > + debug_putstr("Random addr is not allowed. No relocation needed... \n");
> > + return;
> > + }
> > +
>
> It's not clear to me what this is fixing. In aslr.c,
> process_e820_entry() should already make it impossible to select
> max_addr > CONFIG_RANDOMIZE_BASE_MAX_OFFSET. If you're trying to
> detect a non-kaslr boot, I think this is better handled in 1/4 where I
> suggest examining the "output" location before/after
> choose_kernel_location.
>
> -Kees

Hi Kees,

Yes, process_e820_entry() can make sure the choice+output_len <
CONFIG_RANDOMIZE_BASE_MAX_OFFSET, but that can't stop other bootloaders
to put kernel in region above CONFIG_RANDOMIZE_BASE_MAX_OFFSET.

E.g in kdump, we can set crashkernel=256M@1024M in cmdline. Then the 1st
kernel will reserve 256M memory just at 1024M place. So if load kdump
kernel now, the output will be 1024M before choose_kernel_location().
With this value, output won't be changed in choose_kernel_location(),
then it will do decompress(), then call handle_relocations(). Then since
1024 is not equal to LOAD_PHYSICAL_ADDR, it will start relocatoins
handling. And this cause _text stamping into MODULES vaddr range. System
will be exceptional.

Thanks
Baoquan
>
> > /*
> > * Calculate the delta between where vmlinux was linked to load
> > * and where it was actually loaded.
> > --
> > 1.8.5.3
> >
>
>
>
> --
> Kees Cook
> Chrome OS Security