FROMLIST: arm64: mm: support ARCH_MMAP_RND_BITS.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/340)

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.

Bug: 24047224
Signed-off-by: Daniel Cashman <dcashman@android.com>
Signed-off-by: Daniel Cashman <dcashman@google.com>
Change-Id: I0be0bf8b1ed412863f248323e2d86b1df5bf21c6

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 68e3e35f1965e4eac6bc3c62087b4735043cb2aa..579ab688312de4a74ea186d4ba644d19058b8108 100644 (file)
--- 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 ed177475dd8ce6abbee845f40e1cdbe3ebe72adb..4c893b5189ddd027537511c5daaca70f158577c4 100644 (file)
--- 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;
 }