From: dcashman <redacted>

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 <redacted>
---
 Documentation/sysctl/vm.txt | 29 ++++++++++++++++++++
 arch/Kconfig                | 64 +++++++++++++++++++++++++++++++++++++++++++++
 include/linux/mm.h          | 11 ++++++++
 kernel/sysctl.c             | 22 ++++++++++++++++
 mm/mmap.c                   | 12 +++++++++
 5 files changed, 138 insertions(+)

diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index f72370b..d77a81a 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/kernel/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/kernel/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..141823f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -511,6 +511,70 @@ 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/kernel/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
+       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_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/kernel/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..7d39828 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 int mmap_rnd_bits_min;
+extern int mmap_rnd_bits_max;
+extern int mmap_rnd_bits;
+#endif
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+extern int mmap_rnd_compat_bits_min;
+extern int mmap_rnd_compat_bits_max;
+extern int mmap_rnd_compat_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 dc6858d..40e5de6 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           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = &mmap_rnd_bits_min,
+               .extra2         = &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           = 0644,
+               .proc_handler   = proc_dointvec_minmax,
+               .extra1         = &mmap_rnd_compat_bits_min,
+               .extra2         = &mmap_rnd_compat_bits_max,
+       },
+#endif
        { }
 };

diff --git a/mm/mmap.c b/mm/mmap.c
index 2ce04a6..aa49841 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
+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
+#ifdef CONFIG_HAVE_ARCH_MMAP_RND_COMPAT_BITS
+int mmap_rnd_compat_bits_min = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MIN;
+int mmap_rnd_compat_bits_max = CONFIG_ARCH_MMAP_RND_COMPAT_BITS_MAX;