Re: [PATCH] Make LIST_POISON less deadly

From: Andi Kleen
Date: Tue May 20 2008 - 07:25:17 EST

Avi Kivity <avi@xxxxxxxxxxxx> writes:

> 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.

Hmm, thought I had sent a reply earlier, but don't see it so again.
My apologies if you see it twice.

The problem with your address values is that they're non canonical
and will result in a #GP, not #PF and oops handler cannot display
the address which will make them much less obvious.

I would rather use a guaranteed to be unmapped but canonical
address like in the ffffc10000000000 - ffffc1ffffffffff range
so that you still get page faults.

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