FROMLIST: x86: mm: support ARCH_MMAP_RND_BITS.

(cherry picked from commit https://lkml.org/lkml/2015/12/21/339)

x86: arch_mmap_rnd() uses hard-coded values, 8 for 32-bit and 28 for
64-bit, 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: Ic38735a8de2943843a73b5c20855ccfa92513422

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f22b618303a8bc9c3c2f34762f400dbafb..ae3c83e8bde8f5fb252e98c6b07c4a2ba2bed993 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -46,6 +46,8 @@ config X86
        select ARCH_WANTS_DYNAMIC_TASK_STRUCT
        select ARCH_WANT_FRAME_POINTERS
        select ARCH_WANT_IPC_PARSE_VERSION      if X86_32
+       select HAVE_ARCH_MMAP_RND_BITS          if MMU
+       select HAVE_ARCH_MMAP_RND_COMPAT_BITS   if MMU && COMPAT
        select ARCH_WANT_OPTIONAL_GPIOLIB
        select BUILDTIME_EXTABLE_SORT
        select CLKEVT_I8253
@@ -183,6 +185,20 @@ config HAVE_LATENCYTOP_SUPPORT
 config MMU
        def_bool y
 
+config ARCH_MMAP_RND_BITS_MIN
+       default 28 if 64BIT
+       default 8
+
+config ARCH_MMAP_RND_BITS_MAX
+       default 32 if 64BIT
+       default 16
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 8
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 16
+
 config SBUS
        bool
 

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 844b06d67df4da95cec611375d55c05d52884efd..96bd1e2bffafb3b8ffbc28906a2998ad992d50e8 100644 (file)
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -69,14 +69,14 @@ unsigned long arch_mmap_rnd(void)
 {
        unsigned long rnd;
 
-       /*
-        *  8 bits of randomness in 32bit mmaps, 20 address space bits
-        * 28 bits of randomness in 64bit mmaps, 40 address space bits
-        */
        if (mmap_is_ia32())
-               rnd = (unsigned long)get_random_int() % (1<<8);
+#ifdef CONFIG_COMPAT
+               rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_compat_bits) - 1);
+#else
+               rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_bits) - 1);
+#endif
        else
-               rnd = (unsigned long)get_random_int() % (1<<28);
+               rnd = (unsigned long)get_random_int() & ((1 << mmap_rnd_bits) - 1);
 
        return rnd << PAGE_SHIFT;
 }