[PATCH] mm: mmap: allow for the maximum number of bits for randomizing mmap_base by default

[Rafael Aquini]

An ASLR regression was noticed [1] and tracked down to file-mapped areas
being backed by THP in recent kernels. The 21-bit alignment constraint
for such mappings reduces the entropy for randomizing the placement of
64-bit library mappings and breaks ASLR completely for 32-bit libraries.

The reported issue is easily addressed by increasing vm.mmap_rnd_bits
and vm.mmap_rnd_compat_bits. This patch just provides a simple way to
set ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS to their maximum
values allowed by the architecture at build time.

[1] https://zolutal.github.io/aslrnt/

Signed-off-by: Rafael Aquini <aquini@xxxxxxxxxx>
---
 arch/Kconfig | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 975dd22a2dbd..b142f0752dc5 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1046,10 +1046,21 @@ config ARCH_MMAP_RND_BITS_MAX
 config ARCH_MMAP_RND_BITS_DEFAULT
 	int
 
+config FORCE_MAX_MMAP_RND_BITS
+	bool "Force maximum number of bits to use for ASLR of mmap base address"
+	default n
+	help
+	  ARCH_MMAP_RND_BITS and ARCH_MMAP_RND_COMPAT_BITS represent the number
+	  of bits to use for ASLR and if no custom value is assigned (EXPERT)
+	  then the architercure's lower bound (minimum) value is assumed.
+	  This toggle changes that default assumption to assume the arch upper
+	  bound (maximum) value instead.
+
 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
+	default ARCH_MMAP_RND_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
 	default ARCH_MMAP_RND_BITS_MIN
 	depends on HAVE_ARCH_MMAP_RND_BITS
 	help
@@ -1084,6 +1095,7 @@ 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_DEFAULT if ARCH_MMAP_RND_COMPAT_BITS_DEFAULT
+	default ARCH_MMAP_RND_COMPAT_BITS_MAX if FORCE_MAX_MMAP_RND_BITS
 	default ARCH_MMAP_RND_COMPAT_BITS_MIN
 	depends on HAVE_ARCH_MMAP_RND_COMPAT_BITS
 	help
-- 
2.45.1