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

Vivek Goyal
2014-09-09T19:41:32Z

On Sat, Sep 06, 2014 at 06:16:57AM +0800, Baoquan He wrote:
> 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
>

[CC hpa ]

> 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.

Bao,

If you apply your first patch where output_orig == output, then
handle_relocations() will not do anything for x86_64 case and bail
out. That should take care of this issue. Isn't it? And we should
not require this patch.

Thanks
Vivek