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Date: Tue, 25 Jul 2017 00:17:11 +0300
From: Alexander Popov <>
To: Kees Cook <>
Cc: Andrew Morton <>,
 Christoph Lameter <>, Pekka Enberg <>,
 David Rientjes <>, Joonsoo Kim <>,
 "Paul E. McKenney" <>,
 Ingo Molnar <>, Josh Triplett <>,
 Andy Lutomirski <>, Nicolas Pitre <>,
 Tejun Heo <>, Daniel Mack <>,
 Sebastian Andrzej Siewior <>,
 Sergey Senozhatsky <>,
 Helge Deller <>, Rik van Riel <>,, Tycho Andersen <>,,
Subject: Re: [v3] mm: Add SLUB free list pointer obfuscation

>From 86f4f1f6deb76849e00c761fa30eeb479f789c35 Mon Sep 17 00:00:00 2001
From: Alexander Popov <>
Date: Mon, 24 Jul 2017 23:16:28 +0300
Subject: [PATCH 2/2] mm/slub.c: add a naive detection of double free or

On 06.07.2017 03:27, Kees Cook wrote:
> This SLUB free list pointer obfuscation code is modified from Brad
> Spengler/PaX Team's code in the last public patch of grsecurity/PaX based
> on my understanding of the code. Changes or omissions from the original
> code are mine and don't reflect the original grsecurity/PaX code.
> This adds a per-cache random value to SLUB caches that is XORed with
> their freelist pointer address and value. This adds nearly zero overhead
> and frustrates the very common heap overflow exploitation method of
> overwriting freelist pointers. A recent example of the attack is written
> up here:
> This is based on patches by Daniel Micay, and refactored to minimize the
> use of #ifdef.


This is an addition to the SLAB_FREELIST_HARDENED feature. I'm sending it
according the discussion here:

-- >8 --

Add an assertion similar to "fasttop" check in GNU C Library allocator
as a part of SLAB_FREELIST_HARDENED feature. An object added to a singly
linked freelist should not point to itself. That helps to detect some
double free errors (e.g. CVE-2017-2636) without slub_debug and KASAN.

Signed-off-by: Alexander Popov <>
 mm/slub.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index c92d636..f39d06e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -290,6 +290,10 @@ static inline void set_freepointer(struct kmem_cache *s,
void *object, void *fp)
 	unsigned long freeptr_addr = (unsigned long)object + s->offset;

+	BUG_ON(object == fp); /* naive detection of double free or corruption */
 	*(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);


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