linux-kernel - Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support

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Date:	Fri, 29 Feb 2008 13:09:28 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>, Zachary Amsden <zach@...are.com>
Subject: Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support
 - alternatives"

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
>> The patch "x86 - Enhance DEBUG_RODATA support - alternatives" enables 
>> the kernel for writing by clearing X86_CR0_WP allow privileged 
>> writes.  This won't work in a paravirt environment for two reasons:
>>
>>   1. the kernel may not be running in ring 0, so writes will still be
>>      prevented
>>   2. the hypervisor prevents X86_CR0_WP from being cleared anyway (it
>>      GPFs the cr0 update)
>>
>> This crashes on Xen, and it would probably break VMI too.

(lguest too, of course)

>> The only safe way to allow writes is to change the page permissions 
>> (either on the page itself, or create a temporary writable alias for 
>> that page).  Perhaps something you could do it with kmap_atomic.
>>
>
> A properly implemented hypervisor should arguably emulate this.
>
> Doesn't really mean the patch is worth the pain.

No, it would be irritating to implement.

Seems to me that doing the update in a temporary kmap_atomic mapping 
would be a more straightforward way to go, anyway.  How would you 
implement this on a processor without something like X86_CR0_WP?

    J
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