linux-kernel - [PATCH v4 04/10] mips: extract logic for mmap

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Date:	Wed,  4 Mar 2015 13:10:48 -0800
From:	Kees Cook <keescook@...omium.org>
To:	akpm@...ux-foundation.org
Cc:	Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Hector Marco-Gisbert <hecmargi@....es>,
	Ismael Ripoll <iripoll@....es>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux390@...ibm.com, x86@...nel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	"David A. Long" <dave.long@...aro.org>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Arun Chandran <achandran@...sta.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Min-Hua Chen <orca.chen@...il.com>,
	Paul Burton <paul.burton@...tec.com>,
	Alex Smith <alex@...x-smith.me.uk>,
	Markos Chandras <markos.chandras@...tec.com>,
	Vineeth Vijayan <vvijayan@...sta.com>,
	Jeff Bailey <jeffbailey@...gle.com>,
	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Ben Hutchings <ben@...adent.org.uk>,
	Borislav Petkov <bp@...e.de>,
	Jan-Simon Möller <dl9pf@....de>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mips@...ux-mips.org, linuxppc-dev@...ts.ozlabs.org,
	linux-s390@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH v4 04/10] mips: extract logic for mmap_rnd()

In preparation for splitting out ET_DYN ASLR, extract the mmap ASLR
selection into a separate function.

Signed-off-by: Kees Cook <keescook@...omium.org>
---
It seems the entropy gets smaller as the PAGE_SIZE increases. Is this
intentional?
---
 arch/mips/mm/mmap.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index f1baadd56e82..9a4f1f5c1f0e 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c
@@ -142,18 +142,26 @@ unsigned long arch_get_unmapped_area_topdown(struct file *filp,
 			addr0, len, pgoff, flags, DOWN);
 }
 
+static unsigned long mmap_rnd(void)
+{
+	unsigned long rnd;
+
+	rnd = (unsigned long)get_random_int();
+	rnd <<= PAGE_SHIFT;
+	if (TASK_IS_32BIT_ADDR)
+		rnd &= 0xfffffful;
+	else
+		rnd &= 0xffffffful;
+
+	return rnd;
+}
+
 void arch_pick_mmap_layout(struct mm_struct *mm)
 {
 	unsigned long random_factor = 0UL;
 
-	if (current->flags & PF_RANDOMIZE) {
-		random_factor = get_random_int();
-		random_factor = random_factor << PAGE_SHIFT;
-		if (TASK_IS_32BIT_ADDR)
-			random_factor &= 0xfffffful;
-		else
-			random_factor &= 0xffffffful;
-	}
+	if (current->flags & PF_RANDOMIZE)
+		random_factor = mmap_rnd();
 
 	if (mmap_is_legacy()) {
 		mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
-- 
1.9.1

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