[3.16.y-ckt stable] Patch "arm64/mm: Remove hack in mmap randomize layout" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    arm64/mm: Remove hack in mmap randomize layout

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt12.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

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>From 92a2a8f24fe827cf94880bd15edbaf1da9812525 Mon Sep 17 00:00:00 2001
From: Yann Droneaud <ydroneaud at opteya.com>
Date: Mon, 17 Nov 2014 23:02:19 +0000
Subject: arm64/mm: Remove hack in mmap randomize layout
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

commit d6c763afab142a85e4770b4bc2a5f40f256d5c5d upstream.

Since commit 8a0a9bd4db63 ('random: make get_random_int() more
random'), get_random_int() returns a random value for each call,
so comment and hack introduced in mmap_rnd() as part of commit
1d18c47c735e ('arm64: MMU fault handling and page table management')
are incorrects.

Commit 1d18c47c735e seems to use the same hack introduced by
commit a5adc91a4b44 ('powerpc: Ensure random space between stack
and mmaps'), latter copied in commit 5a0efea09f42 ('sparc64: Sharpen
address space randomization calculations.').

But both architectures were cleaned up as part of commit
fa8cbaaf5a68 ('powerpc+sparc64/mm: Remove hack in mmap randomize
layout') as hack is no more needed since commit 8a0a9bd4db63.

So the present patch removes the comment and the hack around
get_random_int() on AArch64's mmap_rnd().

Cc: David S. Miller <davem at davemloft.net>
Cc: Anton Blanchard <anton at samba.org>
Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
Acked-by: Will Deacon <will.deacon at arm.com>
Acked-by: Dan McGee <dpmcgee at gmail.com>
Signed-off-by: Yann Droneaud <ydroneaud at opteya.com>
Signed-off-by: Will Deacon <will.deacon at arm.com>
Cc: Moritz Mühlenhoff <jmm at inutil.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 arch/arm64/mm/mmap.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
index 8ed6cb1a900f..8f7ffffc63e9 100644
--- a/arch/arm64/mm/mmap.c
+++ b/arch/arm64/mm/mmap.c
@@ -47,22 +47,14 @@ static int mmap_is_legacy(void)
 	return sysctl_legacy_va_layout;
 }

-/*
- * Since get_random_int() returns the same value within a 1 jiffy window, we
- * will almost always get the same randomisation for the stack and mmap
- * region. This will mean the relative distance between stack and mmap will be
- * the same.
- *
- * To avoid this we can shift the randomness by 1 bit.
- */
 static unsigned long mmap_rnd(void)
 {
 	unsigned long rnd = 0;

 	if (current->flags & PF_RANDOMIZE)
-		rnd = (long)get_random_int() & (STACK_RND_MASK >> 1);
+		rnd = (long)get_random_int() & STACK_RND_MASK;

-	return rnd << (PAGE_SHIFT + 1);
+	return rnd << PAGE_SHIFT;
 }

 static unsigned long mmap_base(void)