linux-kernel - Re: [PATCH] Make LIST

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Date:	Mon, 19 May 2008 15:01:51 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Make LIST_POISON less deadly


* Avi Kivity <avi@...ranet.com> wrote:

> 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 smaller 
> than the range spanned by pointer values.

nice idea!

i'd suggest a slightly different solution:

> +/*
> + * Define LIST_POISON[12] as pointers that cannot be dereferenced.
> + */
> +#ifdef CONFIG_X86_84
> +#  undef LIST_POISON1
> +#  undef LIST_POISON2
> +#  define LIST_POISON1 ((void *)0x8001000100010001L)
> +#  define LIST_POISON2 ((void *)0x8002000200020002L)
> +#endif

i'd suggest to add an ARCH_ILLEGAL_POINTER define instead, which 
defaults to zero and gets added to pointer-ish poison values. That makes 
it both simpler and also it does not need any include/asm changes 
because ARCH_ILLEGAL_POINTER can be set from the Kconfig space.

hm?

	Ingo
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