[PATCH v3 1/4] mm: mmap: Add new /proc tunable for mmap_base ASLR.

On 11/24/2015 08:40 PM, Michael Ellerman wrote:

On Wed, 2015-11-18 at 15:20 -0800, Daniel Cashman wrote:

quoted

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.

...

quoted

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

Here you support a default which is separate from the minimum.

quoted

+	default ARCH_MMAP_RND_BITS_MIN
+	depends on HAVE_ARCH_MMAP_RND_BITS

...

quoted

+
+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

But here you don't.

Just forgot?

Yes.  Good catch.

I'd like to have a default which is separate from the minimum. That way we can
have a default which is reasonably large, but allow it to be lowered easily if
anything breaks.

Will add it, along w/the documentation cleanup and other changes.

Thank You,
Dan