Re: [PATCH v2 1/2] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Kees Cook <keescook@chromium.org>]

On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@android.com> wrote:
> From: dcashman <dcashman@google.com>
>
> ASLR currently only uses 8 bits to generate the random offset for the
> mmap base address on 32 bit architectures. This value was chosen to
> prevent a poorly chosen value from dividing the address space in such
> a way as to prevent large allocations. This may not be an issue on all
> platforms. Allow the specification of a minimum number of bits so that
> platforms desiring greater ASLR protection may determine where to place
> the trade-off.
>
> Signed-off-by: Daniel Cashman <dcashman@google.com>

I like this, thanks for working on it!

Acked-by: Kees Cook <keescook@chromium.org>

We might end up in situations on some architectures where mappings
might end up crashing into each other, but I think that'll be a
per-arch concern. Being able to set this at all is a great
improvement.

Thanks!

-Kees

> ---
> Changes in v2:
>   - Added HAVE_ARCH_MMAP_RND_BITS as Kconfig boolean selector.
>   - Moved ARCH_MMAP_RND_BITS_MIN, ARCH_MMAP_RND_BITS_MAX, and
>   ARCH_MMAP_RND_BITS declarations to arch/Kconfig instead of relying
>   soley on arch-specific Kconfigs.
>   - Moved definition of mmap_rnd_bits_min, mmap_rnd_bits_max and
>   mmap_rnd_bits to mm/mmap.c instead of relying solely on arch-specific
>   code.
>
>  Documentation/sysctl/kernel.txt | 14 ++++++++++++++
>  arch/Kconfig                    | 29 +++++++++++++++++++++++++++++
>  include/linux/mm.h              |  6 ++++++
>  kernel/sysctl.c                 | 11 +++++++++++
>  mm/mmap.c                       |  6 ++++++
>  5 files changed, 66 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 6fccb69..0d4ca53 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -41,6 +41,7 @@ show up in /proc/sys/kernel:
>  - kptr_restrict
>  - kstack_depth_to_print       [ X86 only ]
>  - l2cr                        [ PPC only ]
> +- mmap_rnd_bits
>  - modprobe                    ==> Documentation/debugging-modules.txt
>  - modules_disabled
>  - msg_next_id                [ sysv ipc ]
> @@ -391,6 +392,19 @@ This flag controls the L2 cache of G3 processor boards. If
>
>  ==============================================================
>
> +mmap_rnd_bits:
> +
> +This value can be used to select the number of bits to use to
> +determine the random offset to the base address of vma regions
> +resulting from mmap allocations on architectures which support
> +tuning address space randomization.  This value will be bounded
> +by the architecture's minimum and maximum supported values.
> +
> +This value can be changed after boot using the
> +/proc/sys/kernel/mmap_rnd_bits tunable
> +
> +==============================================================
> +
>  modules_disabled:
>
>  A toggle value indicating if modules are allowed to be loaded
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 4e949e5..2133973 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -511,6 +511,35 @@ config ARCH_HAS_ELF_RANDOMIZE
>           - arch_mmap_rnd()
>           - arch_randomize_brk()
>
> +config HAVE_ARCH_MMAP_RND_BITS
> +       bool
> +       help
> +         An arch should select this symbol if it supports setting a variable
> +         number of bits for use in establishing the base address for mmap
> +         allocations and provides values for both:
> +         - ARCH_MMAP_RND_BITS_MIN
> +         - ARCH_MMAP_RND_BITS_MAX
> +
> +config ARCH_MMAP_RND_BITS_MIN
> +       int
> +
> +config ARCH_MMAP_RND_BITS_MAX
> +       int
> +
> +config ARCH_MMAP_RND_BITS
> +       int "Number of bits to use for ASLR of mmap base address" if EXPERT
> +       range ARCH_MMAP_RND_BITS_MIN ARCH_MMAP_RND_BITS_MAX
> +       default ARCH_MMAP_RND_BITS_MIN
> +       depends on HAVE_ARCH_MMAP_RND_BITS
> +       help
> +         This value can be used to select the number of bits to use to
> +         determine the random offset to the base address of vma regions
> +         resulting from mmap allocations. This value will be bounded
> +         by the architecture's minimum and maximum supported values.
> +
> +         This value can be changed after boot using the
> +         /proc/sys/kernel/mmap_rnd_bits tunable
> +
>  config HAVE_COPY_THREAD_TLS
>         bool
>         help
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 80001de..ee209c1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -51,6 +51,12 @@ extern int sysctl_legacy_va_layout;
>  #define sysctl_legacy_va_layout 0
>  #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +extern int mmap_rnd_bits_min;
> +extern int mmap_rnd_bits_max;
> +extern int mmap_rnd_bits;
> +#endif
> +
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index e69201d..276da8b 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1139,6 +1139,17 @@ static struct ctl_table kern_table[] = {
>                 .proc_handler   = timer_migration_handler,
>         },
>  #endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +       {
> +               .procname       = "mmap_rnd_bits",
> +               .data           = &mmap_rnd_bits,
> +               .maxlen         = sizeof(mmap_rnd_bits),
> +               .mode           = 0644,
> +               .proc_handler   = proc_dointvec_minmax,
> +               .extra1         = &mmap_rnd_bits_min,
> +               .extra2         = &mmap_rnd_bits_max,
> +       },
> +#endif
>         { }
>  };
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 79bcc9f..264aa8e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -58,6 +58,12 @@
>  #define arch_rebalance_pgtables(addr, len)             (addr)
>  #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
> +int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
> +int mmap_rnd_bits = CONFIG_ARCH_MMAP_RND_BITS;
> +#endif
> +
>  static void unmap_region(struct mm_struct *mm,
>                 struct vm_area_struct *vma, struct vm_area_struct *prev,
>                 unsigned long start, unsigned long end);
> --
> 2.6.0.rc2.230.g3dd15c0
>



-- 
Kees Cook
Chrome OS Security