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Subject[PATCH v7 3/4] arm64: mm: support ARCH_MMAP_RND_BITS.
Date
From: dcashman <dcashman@google.com>

arm64: arch_mmap_rnd() uses STACK_RND_MASK to generate the
random offset for the mmap base address. This value represents a
compromise between increased ASLR effectiveness and avoiding
address-space fragmentation. Replace it with a Kconfig option, which
is sensibly bounded, so that platform developers may choose where to
place this compromise. Keep default values as new minimums.

Signed-off-by: Daniel Cashman <dcashman@android.com>
---
arch/arm64/Kconfig | 29 +++++++++++++++++++++++++++++
arch/arm64/mm/mmap.c | 8 ++++++--
2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 871f217..a4de63e 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -51,6 +51,8 @@ config ARM64
select HAVE_ARCH_JUMP_LABEL
select HAVE_ARCH_KASAN if SPARSEMEM_VMEMMAP && !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
select HAVE_ARCH_KGDB
+ select HAVE_ARCH_MMAP_RND_BITS
+ select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_TRACEHOOK
select HAVE_BPF_JIT
@@ -104,6 +106,33 @@ config ARCH_PHYS_ADDR_T_64BIT
config MMU
def_bool y

+config ARCH_MMAP_RND_BITS_MIN
+ default 14 if ARM64_64K_PAGES
+ default 16 if ARM64_16K_PAGES
+ default 18
+
+# max bits determined by the following formula:
+# VA_BITS - PAGE_SHIFT - 3
+config ARCH_MMAP_RND_BITS_MAX
+ default 19 if ARM64_VA_BITS=36
+ default 24 if ARM64_VA_BITS=39
+ default 27 if ARM64_VA_BITS=42
+ default 30 if ARM64_VA_BITS=47
+ default 29 if ARM64_VA_BITS=48 && ARM64_64K_PAGES
+ default 31 if ARM64_VA_BITS=48 && ARM64_16K_PAGES
+ default 33 if ARM64_VA_BITS=48
+ default 14 if ARM64_64K_PAGES
+ default 16 if ARM64_16K_PAGES
+ default 18
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+ default 7 if ARM64_64K_PAGES
+ default 9 if ARM64_16K_PAGES
+ default 11
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+ default 16
+
config NO_IOPORT_MAP
def_bool y if !PCI

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index ed17747..4c893b5 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -51,8 +51,12 @@ unsigned long arch_mmap_rnd(void)
{
unsigned long rnd;

- rnd = (unsigned long)get_random_int() & STACK_RND_MASK;
-
+#ifdef CONFIG_COMPAT
+ if (test_thread_flag(TIF_32BIT))
+ rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_compat_bits) - 1);
+ else
+#endif
+ rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_bits) - 1);
return rnd << PAGE_SHIFT;
}

--
2.6.0.rc2.230.g3dd15c0


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