Re: [PATCH v4 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

[Kees Cook <keescook@chromium.org>]

On Thu, Nov 26, 2015 at 2:59 PM, Daniel Cashman <dcashman@android.com> wrote:
> ASLR  only uses as few as 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@android.com>

This looks good! Thanks for suffering through the many revision requests. :)

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

-Kees

> ---
>  Documentation/sysctl/vm.txt | 29 +++++++++++++++++++
>  arch/Kconfig                | 68 +++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/mm.h          | 11 ++++++++
>  kernel/sysctl.c             | 22 +++++++++++++++
>  mm/mmap.c                   | 12 ++++++++
>  5 files changed, 142 insertions(+)
>
> diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
> index f72370b..ee763f3 100644
> --- a/Documentation/sysctl/vm.txt
> +++ b/Documentation/sysctl/vm.txt
> @@ -42,6 +42,8 @@ Currently, these files are in /proc/sys/vm:
>  - min_slab_ratio
>  - min_unmapped_ratio
>  - mmap_min_addr
> +- mmap_rnd_bits
> +- mmap_rnd_compat_bits
>  - nr_hugepages
>  - nr_overcommit_hugepages
>  - nr_trim_pages         (only if CONFIG_MMU=n)
> @@ -485,6 +487,33 @@ against future potential kernel bugs.
>
>  ==============================================================
>
> +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/vm/mmap_rnd_bits tunable
> +
> +==============================================================
> +
> +mmap_rnd_compat_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 for applications run in
> +compatibility mode 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/vm/mmap_rnd_compat_bits tunable
> +
> +==============================================================
> +
>  nr_hugepages
>
>  Change the minimum size of the hugepage pool.
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 4e949e5..237f1c5 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -511,6 +511,74 @@ 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_DEFAULT
> +       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_DEFAULT if ARCH_MMAP_RND_BITS_DEFAULT
> +       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/vm/mmap_rnd_bits tunable
> +
> +config HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +       bool
> +       help
> +         An arch should select this symbol if it supports running applications
> +         in compatibility mode, 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_COMPAT_BITS_MIN
> +         - ARCH_MMAP_RND_COMPAT_BITS_MAX
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MIN
> +       int
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_MAX
> +       int
> +
> +config ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
> +       int
> +
> +config ARCH_MMAP_RND_COMPAT_BITS
> +       int "Number of bits to use for ASLR of mmap base address for compatible applications" if EXPERT
> +       range ARCH_MMAP_RND_COMPAT_BITS_MIN ARCH_MMAP_RND_COMPAT_BITS_MAX
> +       default ARCH_MMAP_RND_COMPAT_BITS_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
> +       default ARCH_MMAP_RND_COMPAT_BITS_MIN
> +       depends on HAVE_ARCH_MMAP_RND_COMPAT_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 for compatible applications 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/vm/mmap_rnd_compat_bits tunable
> +
>  config HAVE_COPY_THREAD_TLS
>         bool
>         help
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00bad77..6f6dd6e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -51,6 +51,17 @@ extern int sysctl_legacy_va_layout;
>  #define sysctl_legacy_va_layout 0
>  #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +extern const int mmap_rnd_bits_min;
> +extern const int mmap_rnd_bits_max;
> +extern int mmap_rnd_bits __read_mostly;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +extern const int mmap_rnd_compat_bits_min;
> +extern const int mmap_rnd_compat_bits_max;
> +extern int mmap_rnd_compat_bits __read_mostly;
> +#endif
> +
>  #include <asm/page.h>
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index dc6858d..a9db0cf 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1568,6 +1568,28 @@ static struct ctl_table vm_table[] = {
>                 .mode           = 0644,
>                 .proc_handler   = proc_doulongvec_minmax,
>         },
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +       {
> +               .procname       = "mmap_rnd_bits",
> +               .data           = &mmap_rnd_bits,
> +               .maxlen         = sizeof(mmap_rnd_bits),
> +               .mode           = 0600,
> +               .proc_handler   = proc_dointvec_minmax,
> +               .extra1         = (void *) &mmap_rnd_bits_min,
> +               .extra2         = (void *) &mmap_rnd_bits_max,
> +       },
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +       {
> +               .procname       = "mmap_rnd_compat_bits",
> +               .data           = &mmap_rnd_compat_bits,
> +               .maxlen         = sizeof(mmap_rnd_compat_bits),
> +               .mode           = 0600,
> +               .proc_handler   = proc_dointvec_minmax,
> +               .extra1         = (void *) &mmap_rnd_compat_bits_min,
> +               .extra2         = (void *) &mmap_rnd_compat_bits_max,
> +       },
> +#endif
>         { }
>  };
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a6..fe3816c 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -58,6 +58,18 @@
>  #define arch_rebalance_pgtables(addr, len)             (addr)
>  #endif
>
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_BITS
> +const int mmap_rnd_bits_min = CONFIG_ARCH_MMAP_RND_BITS_MIN;
> +const int mmap_rnd_bits_max = CONFIG_ARCH_MMAP_RND_BITS_MAX;
> +int mmap_rnd_bits __read_mostly = CONFIG_ARCH_MMAP_RND_BITS;
> +#endif
> +#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
> +const int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
> +const int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;
> +int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_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 & Brillo Security