From 8b8addf891de8a00e4d39fc32f93f7c5eb8feceb Mon Sep 17 00:00:00 2001

From: Hector Marco-Gisbert <hecmargi@upv.es>

Date: Thu, 10 Mar 2016 20:51:00 +0100

Subject: [PATCH] x86/mm/32: Enable full randomization on i386 and X86_32

Currently on i386 and on X86_64 when emulating X86_32 in legacy mode, only

the stack and the executable are randomized but not other mmapped files

(libraries, vDSO, etc.). This patch enables randomization for the

libraries, vDSO and mmap requests on i386 and in X86_32 in legacy mode.

By default on i386 there are 8 bits for the randomization of the libraries,

vDSO and mmaps which only uses 1MB of VA.

This patch preserves the original randomness, using 1MB of VA out of 3GB or

4GB. We think that 1MB out of 3GB is not a big cost for having the ASLR.

The first obvious security benefit is that all objects are randomized (not

only the stack and the executable) in legacy mode which highly increases

the ASLR effectiveness, otherwise the attackers may use these

non-randomized areas. But also sensitive setuid/setgid applications are

more secure because currently, attackers can disable the randomization of

these applications by setting the ulimit stack to "unlimited". This is a

very old and widely known trick to disable the ASLR in i386 which has been

allowed for too long.

Another trick used to disable the ASLR was to set the ADDR_NO_RANDOMIZE

personality flag, but fortunately this doesn't work on setuid/setgid

applications because there is security checks which clear Security-relevant

flags.

This patch always randomizes the mmap_legacy_base address, removing the

possibility to disable the ASLR by setting the stack to "unlimited".

Signed-off-by: Hector Marco-Gisbert <hecmargi@upv.es>

Acked-by: Ismael Ripoll Ripoll <iripoll@upv.es>

Acked-by: Kees Cook <keescook@chromium.org>

Acked-by: Arjan van de Ven <arjan@linux.intel.com>

Cc: Linus Torvalds <torvalds@linux-foundation.org>

Cc: Peter Zijlstra <peterz@infradead.org>

Cc: Thomas Gleixner <tglx@linutronix.de>

Cc: akpm@linux-foundation.org

Cc: kees Cook <keescook@chromium.org>

Link: http://lkml.kernel.org/r/1457639460-5242-1-git-send-email-hecmargi@upv.es

Signed-off-by: Ingo Molnar <mingo@kernel.org>

---

 arch/x86/mm/mmap.c | 14 +-------------

 1 file changed, 1 insertion(+), 13 deletions(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c

index 96bd1e2..389939f 100644

--- a/arch/x86/mm/mmap.c

+++ b/arch/x86/mm/mmap.c

@@ -94,18 +94,6 @@ static unsigned long mmap_base(unsigned long rnd)

 }

 /*

- * Bottom-up (legacy) layout on X86_32 did not support randomization, X86_64

- * does, but not when emulating X86_32

- */

-static unsigned long mmap_legacy_base(unsigned long rnd)

-{

-	if (mmap_is_ia32())

-		return TASK_UNMAPPED_BASE;

-	else

-		return TASK_UNMAPPED_BASE + rnd;

-}

-

-/*

  * This function, called very early during the creation of a new

  * process VM image, sets up which VM layout function to use:

  */

@@ -116,7 +104,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm)

 	if (current->flags & PF_RANDOMIZE)

 		random_factor = arch_mmap_rnd();

-	mm->mmap_legacy_base = mmap_legacy_base(random_factor);

+	mm->mmap_legacy_base = TASK_UNMAPPED_BASE + random_factor;

 	if (mmap_is_legacy()) {

 		mm->mmap_base = mm->mmap_legacy_base;

-- 

2.7.3