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

Rik van Riel
2017-06-21T12:08:03Z

On Tue, 2017-06-20 at 22:58 -0700, Kees Cook wrote:
> The ELF_ET_DYN_BASE position was originally intended to keep loaders
> away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2
> /bin/cat" might cause the subsequent load of /bin/cat into where the
> loader had been loaded.) With the advent of PIE (ET_DYN binaries with
> an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since
> the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE
> is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial
> portion of the address space is unused.
>
> For 32-bit tasks when RLIMIT_STACK is set to RLIM_INFINITY, programs
> are loaded below the mmap region. This means they can be made to
> collide
> (CVE-2017-1000370) or nearly collide (CVE-2017-1000371) with
> pathological
> stack regions. Lowering ELF_ET_DYN_BASE solves both by moving
> programs
> above the mmap region in all cases, and will now additionally avoid
> programs falling back to the mmap region by enforcing MAP_FIXED for
> program loads (i.e. if it would have collided with the stack, now it
> will fail to load instead of falling back to the mmap region).
>
> To allow for a lower ELF_ET_DYN_BASE, loaders (ET_DYN without INTERP)
> are loaded into the mmap region, leaving space available for either
> an
> ET_EXEC binary with a fixed location or PIE being loaded into mmap by
> the
> loader. Only PIE programs are loaded offset from ELF_ET_DYN_BASE,
> which
> means architectures can now safely lower their values without risk of
> loaders colliding with their subsequently loaded programs.
>
> Thanks go to PaX for inspiration on how to approach this solution.
>
> Fixes: d1fd836dcf00 ("mm: split ET_DYN ASLR from mmap ASLR")
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/include/asm/elf.h |  8 ++------
>  fs/binfmt_elf.c            | 38 +++++++++++++++++++++++++++++++-----
> --
>  2 files changed, 33 insertions(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index e8ab9a46bc68..46549973ea98 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -245,12 +245,8 @@ extern int force_personality32;
>  #define CORE_DUMP_USE_REGSET
>  #define ELF_EXEC_PAGESIZE 4096
>
> -/* This is the location that an ET_DYN program is loaded if
> exec'ed.  Typical
> -   use of this is to invoke "./ld.so someprog" to test out a new
> version of
> -   the loader.  We need to make sure that it is out of the way of
> the program
> -   that it will "exec", and that there is sufficient room for the
> brk.  */
> -
> -#define ELF_ET_DYN_BASE (TASK_SIZE / 3 * 2)
> +/* 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)

> +++ 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 :)