linux-kernel - [PATCH v7 4/4] x86: mm: support ARCH_MMAP_RND

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Message-Id: <1450718817-7082-5-git-send-email-dcashman@android.com>
Date:	Mon, 21 Dec 2015 09:26:57 -0800
From:	Daniel Cashman <dcashman@...roid.com>
To:	linux-kernel@...r.kernel.org
Cc:	linux@....linux.org.uk, akpm@...ux-foundation.org,
	keescook@...omium.org, mingo@...nel.org,
	linux-arm-kernel@...ts.infradead.org, corbet@....net,
	dzickus@...hat.com, ebiederm@...ssion.com, xypron.glpk@....de,
	jpoimboe@...hat.com, kirill.shutemov@...ux.intel.com,
	n-horiguchi@...jp.nec.com, aarcange@...hat.com, mgorman@...e.de,
	tglx@...utronix.de, rientjes@...gle.com, linux-mm@...ck.org,
	linux-doc@...r.kernel.org, salyzyn@...roid.com, jeffv@...gle.com,
	nnk@...gle.com, catalin.marinas@....com, will.deacon@....com,
	hpa@...or.com, x86@...nel.org, hecmargi@....es, bp@...e.de,
	dcashman@...gle.com, arnd@...db.de, jonathanh@...dia.com,
	Daniel Cashman <dcashman@...roid.com>
Subject: [PATCH v7 4/4] x86: mm: support ARCH_MMAP_RND_BITS.

From: dcashman <dcashman@...gle.com>

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@...roid.com>
---
 arch/x86/Kconfig   | 16 ++++++++++++++++
 arch/x86/mm/mmap.c | 12 ++++++------
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index db3622f..1eeccb3 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -82,6 +82,8 @@ 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
@@ -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 844b06d..96bd1e2 100644
--- 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;
 }
-- 
2.6.0.rc2.230.g3dd15c0

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