CVE-2016-3672: 7PK - Security Features (CWE-254)
value: The arch_pick_mmap_layout function in arch/x86/mm/mmap.c in the Linux kernel through 4.5.2 does not properly randomize the legacy base address, which makes it easier for local users to defeat the intended restrictions on the ADDR_NO_RANDOMIZE flag, and bypass the ASLR protection mechanism for a setuid or setgid program, by disabling stack-consumption resource limits.
CWE description: Software security is not security software. Here we're concerned with topics like authentication, access control, confidentiality, cryptography, and privilege management.
CVSS v3 base score: 7.8
Published: 2016-04-27T17:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: arch/x86/mm/mmap.c
Commit date: 2016-03-11 09:53:19 +0100
Commit message: 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>

Diff: diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 96bd1e2bffafb..389939f74dd5a 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -91,22 +91,10 @@ static unsigned long mmap_base(unsigned long rnd)
 		gap = MAX_GAP;
 
 	return PAGE_ALIGN(TASK_SIZE - gap - 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:
  */
 void arch_pick_mmap_layout(struct mm_struct *mm)
@@ -114,11 +102,11 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 	unsigned long random_factor = 0UL;
 
 	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;
 		mm->get_unmapped_area = arch_get_unmapped_area;
 	} else {

Vulnerable code: /*
 * 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;
}

	mm->mmap_legacy_base = mmap_legacy_base(random_factor);
Fixed code: 	mm->mmap_legacy_base = TASK_UNMAPPED_BASE + random_factor;