[Xen-devel] [PATCH] x86: (allow to) override LIST_POISON*

["Jan Beulich" <JBeulich@xxxxxxxx>]

Having these point into space not controlled by the hypervisor provides
an unnecessary attack surface. Allow architectures to override them and
utilize that override to make them non-canonical addresses (thus
causing #GP rather than #PF when dereferenced).

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The security aspect of this makes Andrew and me think this should be
considered for 4.5 despite it not fixing an actual bug.

--- a/xen/include/asm-x86/config.h
+++ b/xen/include/asm-x86/config.h
@@ -106,6 +106,10 @@
 /* Return value for zero-size _xmalloc(), distinguished from NULL. */
 #define ZERO_BLOCK_PTR ((void *)0xBAD0BAD0BAD0BAD0UL)
 
+/* Override include/xen/list.h to make these non-canonical addresses. */
+#define LIST_POISON1  ((void *)0x0100100100100100UL)
+#define LIST_POISON2  ((void *)0x0200200200200200UL)
+
 #ifndef __ASSEMBLY__
 extern unsigned long trampoline_phys;
 #define bootsym_phys(sym)                                 \
--- a/xen/include/xen/list.h
+++ b/xen/include/xen/list.h
@@ -10,12 +10,15 @@
 #include <xen/lib.h>
 #include <asm/system.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.
+/*
+ * These are non-NULL pointers that will result in faults under normal
+ * circumstances, used to verify that nobody uses non-initialized list
+ * entries. Architectures can override these.
  */
+#ifndef LIST_POISON1
 #define LIST_POISON1  ((void *) 0x00100100)
 #define LIST_POISON2  ((void *) 0x00200200)
+#endif
 
 /*
  * Simple doubly linked list implementation.




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