On Tue, Nov 3, 2015 at 10:10 AM, Daniel Cashman <dcashman@xxxxxxxxxxx> wrote:
> From: dcashman <dcashman@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
I like this, thanks for working on it!
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
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
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