Re: [kernel-hardening] [PATCH] [RFC] binfmt_elf: Use ELF_ET_DYN_BASE only for PIE

Daniel Micay
2017-06-21T17:32:19Z

On Wed, 2017-06-21 at 10:28 -0700, Kees Cook wrote:
> On Wed, Jun 21, 2017 at 10:27 AM, Daniel Micay <danielmicay@gmail.com>
> wrote:
> > On Wed, 2017-06-21 at 10:23 -0700, Kees Cook wrote:
> > > On Wed, Jun 21, 2017 at 5:07 AM, Rik van Riel <riel@redhat.com>
> > > wrote:
> > > > On Tue, 2017-06-20 at 22:58 -0700, Kees Cook wrote:
> > > > > +/* This is the base location for PIE (ET_DYN with INTERP)
> > > > > loads.
> > > > > */
> > > > > +#define ELF_ET_DYN_BASE 0x400000UL
> > > >
> > > > This value is good for 32 bit binaries, but for 64
> > > > bit binaries you probably want to put ELF_ET_DYN_BASE
> > > > at 4GB or higher.
> > > >
> > > > The latter is necessary because Android uses the
> > > > lower 4GB of address space for its JVM runtime,
> > > > with 32 bit pointers inside that part of the otherwise
> > > > 64 bit address space.
> > > >
> > > > In other words:
> > > >
> > > > #define ELF_ET_DYN_BASE (mmap_is_ia32() ? 0x400000UL :
> > > > 0x100000000UL)
> > >
> > > Ah, interesting. Okay, that should be fine. I'll adjust it.
> > >
> > > > > +++ b/fs/binfmt_elf.c
> > > > >
> > > > > + * Therefore, programs are loaded offset
> > > > > from
> > > > > + * ELF_ET_DYN_BASE and loaders are
> > > > > loaded
> > > > > into the
> > > > > + * independently randomized mmap region
> > > > > (0
> > > > > load_bias
> > > > > + * without MAP_FIXED).
> > > > > + */
> > > > > + if (elf_interpreter) {
> > > > > + load_bias = ELF_ET_DYN_BASE;
> > > > > + if (current->flags &
> > > > > PF_RANDOMIZE)
> > > > > + load_bias +=
> > > > > arch_mmap_rnd();
> > > > > + elf_flags |= MAP_FIXED;
> > > > > + } else
> > > > > + load_bias = 0;
> > > > > +
> > > > > + load_bias -= vaddr;
> > > >
> > > > I like this, and the big comment telling people how it
> > > > works :)
> > >
> > > Thanks! It looks like your patch for commenting load_bias never
> > > got
> > > picked up, so I've added some more comments for that and some
> > > other
> > > things too. (Mostly for all the stuff I have to read a few times
> > > when
> > > I look at this code.)
> > >
> > > -Kees
> > >
> >
> > The stack rlimit calculation fix for space potentially lost to ASLR
> > is
> > probably still needed too, right?
>
> Yes. Was that picked up by akpm already?
>
> -Kees

I think it was dropped when the ET_DYN changes were dropped.