x86: mm: support ARCH_MMAP_RND_BITS

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.

Signed-off-by: Daniel Cashman <dcashman@google.com>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Mark Salyzyn <salyzyn@android.com>
Cc: Jeff Vander Stoep <jeffv@google.com>
Cc: Nick Kralevich <nnk@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Hector Marco-Gisbert <hecmargi@upv.es>
Cc: Borislav Petkov <bp@suse.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 5d2293417946d1b4451b178efeef3746c1e2bc02..24f362bf3ec632d117429061785b6f5865d441e1 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -83,6 +83,8 @@ config X86
        select HAVE_ARCH_KASAN                  if X86_64 && SPARSEMEM_VMEMMAP
        select HAVE_ARCH_KGDB
        select HAVE_ARCH_KMEMCHECK
+       select HAVE_ARCH_MMAP_RND_BITS          if MMU
+       select HAVE_ARCH_MMAP_RND_COMPAT_BITS   if MMU && COMPAT
        select HAVE_ARCH_SECCOMP_FILTER
        select HAVE_ARCH_SOFT_DIRTY             if X86_64
        select HAVE_ARCH_TRACEHOOK
@@ -184,6 +186,20 @@ 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;
 }