Re: [PATCH] x86: (allow to) override LIST_POISON*

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 14/11/14 14:52, Jan Beulich wrote:
> 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@citrix.com>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

> ---
> 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.
>
>
>