[PATCH v3] powerpc: mm: support ARCH_MMAP_RND_BITS

[Bhupesh Sharma <bhsharma@redhat.com>]

powerpc arch_mmap_rnd() currently uses hard-coded values - (23-PAGE_SHIFT) for
32-bit and (30-PAGE_SHIFT) for 64-bit, to generate the random offset
for the mmap base address for a ASLR ELF.

This patch makes sure that powerpc mmap arch_mmap_rnd() implementation
is similar to other ARCHs (like x86, arm64) and uses mmap_rnd_bits
and helpers to generate the mmap address randomization.

The maximum and minimum randomization range values represent
a compromise between increased ASLR effectiveness and avoiding
address-space fragmentation.

Using the Kconfig option and suitable /proc tunable, platform
developers may choose where to place this compromise.

Also this patch keeps the default values as new minimums.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
* Changes since v2:
v2 can be seen here (https://patchwork.kernel.org/patch/9551509/)
    - Changed a few minimum and maximum randomization ranges as per Michael's suggestion.
    - Corrected Kees's email address in the Reviewed-by line.
    - Added further comments in kconfig to explain how the address ranges were worked out.

* Changes since v1:
v1 can be seen here (https://lists.ozlabs.org/pipermail/linuxppc-dev/2017-February/153594.html)
    - No functional change in this patch.
    - Dropped PATCH 2/2 from v1 as recommended by Kees Cook.

 arch/powerpc/Kconfig   | 44 ++++++++++++++++++++++++++++++++++++++++++++
 arch/powerpc/mm/mmap.c |  7 ++++---
 2 files changed, 48 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 97a8bc8..84aae67 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -22,6 +22,48 @@ config MMU
 	bool
 	default y
 
+# min bits determined by the following formula:
+# VA_BITS - PAGE_SHIFT - CONSTANT
+# where,
+# 	VA_BITS = 46 bits for 64BIT and 4GB - 1 Page = 31 bits for 32BIT
+# 	CONSTANT = 16 for 64BIT and 8 for 32BIT
+config ARCH_MMAP_RND_BITS_MIN
+       default 5 if PPC_256K_PAGES && 32BIT  # 31 - 18 - 8 = 5
+       default 7 if PPC_64K_PAGES && 32BIT   # 31 - 16 - 8 = 7
+       default 9 if PPC_16K_PAGES && 32BIT   # 31 - 14 - 8 = 9
+       default 11 if PPC_4K_PAGES && 32BIT   # 31 - 12 - 8 = 11
+       default 12 if PPC_256K_PAGES && 64BIT # 46 - 18 - 16 = 12
+       default 14 if PPC_64K_PAGES && 64BIT  # 46 - 16 - 16 = 14
+       default 16 if PPC_16K_PAGES && 64BIT  # 46 - 14 - 16 = 16
+       default 18 if PPC_4K_PAGES && 64BIT   # 46 - 12 - 16 = 18
+
+# max bits determined by the following formula:
+# VA_BITS - PAGE_SHIFT - CONSTANT
+# where, 
+# 	VA_BITS = 46 bits for 64BIT, and 4GB - 1 Page = 31 bits for 32BIT
+# 	CONSTANT = 2, both for 64BIT and 32BIT
+config ARCH_MMAP_RND_BITS_MAX
+       default 11 if PPC_256K_PAGES && 32BIT # 31 - 18 - 2 = 11
+       default 13 if PPC_64K_PAGES && 32BIT  # 31 - 16 - 2 = 13
+       default 15 if PPC_16K_PAGES && 32BIT  # 31 - 14 - 2 = 15
+       default 17 if PPC_4K_PAGES && 32BIT   # 31 - 12 - 2 = 17
+       default 26 if PPC_256K_PAGES && 64BIT # 46 - 18 - 2 = 26
+       default 28 if PPC_64K_PAGES && 64BIT  # 46 - 16 - 2 = 28
+       default 30 if PPC_16K_PAGES && 64BIT  # 46 - 14 - 2 = 30
+       default 32 if PPC_4K_PAGES && 64BIT   # 46 - 12 - 2 = 32
+
+config ARCH_MMAP_RND_COMPAT_BITS_MIN
+       default 5 if PPC_256K_PAGES
+       default 7 if PPC_64K_PAGES
+       default 9 if PPC_16K_PAGES
+       default 11
+
+config ARCH_MMAP_RND_COMPAT_BITS_MAX
+       default 11 if PPC_256K_PAGES
+       default 13 if PPC_64K_PAGES
+       default 15 if PPC_16K_PAGES 
+       default 17
+
 config HAVE_SETUP_PER_CPU_AREA
 	def_bool PPC64
 
@@ -142,6 +184,8 @@ config PPC
 	select HAVE_IRQ_EXIT_ON_IRQ_STACK
 	select HAVE_KERNEL_GZIP
 	select HAVE_KPROBES
+	select HAVE_ARCH_MMAP_RND_BITS
+	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
 	select HAVE_KRETPROBES
 	select HAVE_LIVEPATCH			if HAVE_DYNAMIC_FTRACE_WITH_REGS
 	select HAVE_MEMBLOCK
diff --git a/arch/powerpc/mm/mmap.c b/arch/powerpc/mm/mmap.c
index a5d9ef5..92a9355 100644
--- a/arch/powerpc/mm/mmap.c
+++ b/arch/powerpc/mm/mmap.c
@@ -61,11 +61,12 @@ unsigned long arch_mmap_rnd(void)
 {
 	unsigned long rnd;
 
-	/* 8MB for 32bit, 1GB for 64bit */
+#ifdef CONFIG_COMPAT
 	if (is_32bit_task())
-		rnd = get_random_long() % (1<<(23-PAGE_SHIFT));
+		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
 	else
-		rnd = get_random_long() % (1UL<<(30-PAGE_SHIFT));
+#endif
+		rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
 
 	return rnd << PAGE_SHIFT;
 }
-- 
2.7.4