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

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Message-ID: <4CC54EB8.9020604@redhat.com>
Date:	Mon, 25 Oct 2010 11:32:40 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	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>,
	Jan Kiszka <jan.kiszka@...mens.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

  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.

-- 
error compiling committee.c: too many arguments to function

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