linux-kernel - [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE

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Message-Id: <1479207422-6535-1-git-send-email-mpe@ellerman.id.au>
Date:   Tue, 15 Nov 2016 21:57:02 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     akpm@...ux-foundation.org
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        keescook@...omium.org
Subject: [PATCH] slab: Add POISON_POINTER_DELTA to ZERO_SIZE_PTR

POISON_POINTER_DELTA is defined in poison.h, and is intended to be used
to shift poison values so that they don't alias userspace.

We should add it to ZERO_SIZE_PTR so that attackers can't use
ZERO_SIZE_PTR as a way to get a pointer to userspace.

Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 include/linux/slab.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 084b12bad198..17ddd7aea2dd 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -12,6 +12,7 @@
 #define	_LINUX_SLAB_H
 
 #include <linux/gfp.h>
+#include <linux/poison.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
 
@@ -109,7 +110,7 @@
  * ZERO_SIZE_PTR can be passed to kfree though in the same way that NULL can.
  * Both make kfree a no-op.
  */
-#define ZERO_SIZE_PTR ((void *)16)
+#define ZERO_SIZE_PTR ((void *)(16 + POISON_POINTER_DELTA))
 
 #define ZERO_OR_NULL_PTR(x) ((unsigned long)(x) <= \
 				(unsigned long)ZERO_SIZE_PTR)
-- 
2.7.4