[PATCH] Make LIST_POISON less deadly (v3)

From: Avi Kivity
Date: Tue May 20 2008 - 07:39:37 EST

The list macros use LIST_POISON1 and LIST_POISON2 as undereferencable
pointers in order to trap erronous use of freed list_heads. Unfortunately
userspace can arrange for those pointers to actually be dereferencable,
potentially turning an oops to an expolit.

To avoid this allow architectures (currently x86_64 only) to override
the default values for these pointers with truly-undereferncable values.
This is easy on x86_64 as the virtual address space is large and contains
unmapped ranges.

Signed-off-by: Avi Kivity <avi@xxxxxxxxxxxx>

Changes since v1:
- add CONFIG_ILLEGAL_POINTER_VALUE instead of new header <asm/poison.h>

Changes since v2:
- switched the range from an uncanonical address to a canonical unmapped
address, since uncannonical addresses generate #GP instead of #PF, losing
the faulting address.

arch/x86/Kconfig | 5 +++++
include/linux/poison.h | 10 ++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index fe361ae..54d4e36 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1021,6 +1021,11 @@ config ARCH_MEMORY_PROBE
def_bool X86_64

+ hex
+ default 0 if X86_32
+ default 0xffffc10000000000 if X86_64
source "mm/Kconfig"

config HIGHPTE
diff --git a/include/linux/poison.h b/include/linux/poison.h
index 9f31683..0d105a5 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -1,14 +1,20 @@

/********** include/linux/list.h **********/
* These are non-NULL pointers that will result in page faults
* under normal circumstances, used to verify that nobody uses
* non-initialized list entries.
-#define LIST_POISON1 ((void *) 0x00100100)
-#define LIST_POISON2 ((void *) 0x00200200)
+#define LIST_POISON1 ((void *) 0x00100100 + POISON_POINTER_DELTA)
+#define LIST_POISON2 ((void *) 0x00200200 + POISON_POINTER_DELTA)

/********** include/linux/timer.h **********/

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