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Date: Tue, 28 Feb 2017 15:52:26 -0800
From: Kees Cook <keescook@...omium.org>
To: Andy Lutomirski <luto@...capital.net>
Cc: "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, 
	Mark Rutland <mark.rutland@....com>, Andy Lutomirski <luto@...nel.org>, Hoeun Ryu <hoeun.ryu@...il.com>, 
	PaX Team <pageexec@...email.hu>, Emese Revfy <re.emese@...il.com>, 
	Russell King <linux@...linux.org.uk>, X86 ML <x86@...nel.org>
Subject: Re: [RFC][PATCH 4/8] x86: Implement __arch_rare_write_map/unmap()

On Tue, Feb 28, 2017 at 2:54 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Feb 28, 2017 at 1:35 PM, Kees Cook <keescook@...omium.org> wrote:
>>> Can't we at least make this:
>>>
>>> struct rare_write_mapping {
>>>   void *addr;
>>>   struct arch_rare_write_state arch_state;
>>> };
>>>
>>> static inline struct rare_write_mapping __arch_rare_write_map(void
>>> *addr, size_t len);
>>> static inline void __arch_rare_write_unmap(struct rare_write_mapping mapping);
>>
>> How do you envision this working with the more complex things I
>> included in the latter patches of the series, namely doing linked list
>> updates across multiple structure instances, etc?
>>
>> ie, poor list manipulation pseudo-code:
>>
>> turn off read-only;
>> struct_one->next = struct_tree->node;
>> struct_three->prev = struct_one->node;
>> struct_two->prev = struct_two->next = NULL;
>> turn on read-only;
>>
>> That's three separate memory areas involved...
>
> Fair enough.  That being said, how is this supposed to work on
> architectures that don't have a global "disable write protection" bit?
> Surely these architectures exist.

I don't know. :) That's part of the reason for putting up this series:
looking to see what's possible on other architectures. It's not clear
to me what arm64 can do, for example. Without domains there didn't
seem to be an obvious global override. My intention is to make sure we
get a viable interface for the architectures that are interested in
these kinds of self-protection thingies. :)

-Kees

-- 
Kees Cook
Pixel Security

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