Re: [v3] mm: Add SLUB free list pointer obfuscation
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Message ID 2a30f7bf-601b-f442-9664-7de5a1501206@linux.com
State New
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Commit Message

Alexander Popov July 25, 2017, 9:42 a.m. UTC
From 86f4f1f6deb76849e00c761fa30eeb479f789c35 Mon Sep 17 00:00:00 2001
From: Alexander Popov <alex.popov@linux.com>
Date: Mon, 24 Jul 2017 23:16:28 +0300
Subject: [PATCH 2/2] mm/slub.c: add a naive detection of double free or
 corruption

On 25.07.2017 00:17, Alexander Popov wrote:
> 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: http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit
>>
>> This is based on patches by Daniel Micay, and refactored to minimize the
>> use of #ifdef.
> 
> Hello!
> 
> This is an addition to the SLAB_FREELIST_HARDENED feature. I'm sending it
> according the discussion here:
> http://www.openwall.com/lists/kernel-hardening/2017/07/17/9

In my previous message my email client wrapped one line and corrupted the patch.
Excuse me for that. See the fixed patch below.

-- >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 <alex.popov@linux.com>
---
 mm/slub.c | 4 ++++
 1 file changed, 4 insertions(+)

Patch
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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;

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