[PATCH v7 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

Daniel Cashman
2015-12-21T17:27:41Z

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>
---
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..ba1b626 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, has MMU enabled 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, has MMU
+ enabled 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..d5e7e24 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