linux-kernel - Re: [PATCH RFC] kvm: write protect memory after slot swap

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Date:	Wed, 24 Nov 2010 20:16:43 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Avi Kivity <avi@...hat.com>
CC:	"Michael S. Tsirkin" <mst@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Gleb Natapov <gleb@...hat.com>,
	Sheng Yang <sheng@...ux.intel.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: write protect memory after slot swap

Am 25.10.2010 11:32, Avi Kivity wrote:
>  On 10/25/2010 03:21 AM, Michael S. Tsirkin wrote:
>> I have observed the following bug trigger:
>>
>> 1. userspace calls GET_DIRTY_LOG
>> 2. kvm_mmu_slot_remove_write_access is called and makes a page ro
>> 3. page fault happens and makes the page writeable
>>     fault is logged in the bitmap appropriately
>> 4. kvm_vm_ioctl_get_dirty_log swaps slot pointers
>>
>> a lot of time passes
>>
>> 5. guest writes into the page
>> 6. userspace calls GET_DIRTY_LOG
>>
>> At point (5), bitmap is clean and page is writeable,
>> thus, guest modification of memory is not logged
>> and GET_DIRTY_LOG returns an empty bitmap.
>>
>> The rule is that all pages are either dirty in the current bitmap,
>> or write-protected, which is violated here.
>>
>> It seems that just moving kvm_mmu_slot_remove_write_access down
>> to after the slot pointer swap should fix this bug.
>>
>> Warning: completely untested.
>> Please comment.
>> Note: fix will be needed for -stable etc.
> 
> Excellent catch, I stared at this code for a while and didn't see the
> bug.  Patch applied.
> 

This patch was marked KVM-stable on commit, but it did not make into any
stable branch thus also none of the recent releases. Please fix (for
2.6.36 now).

Thanks,
Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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