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

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Message-Id: <20101025162734.fdc02309.yoshikawa.takuya@oss.ntt.co.jp>
Date:	Mon, 25 Oct 2010 16:27:34 +0900
From:	Takuya Yoshikawa <yoshikawa.takuya@....ntt.co.jp>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Avi Kivity <avi@...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>,
	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 Mon, 25 Oct 2010 03:21:24 +0200
"Michael S. Tsirkin" <mst@...hat.com> 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

This may be the reason why my commit is a corruption magnifier.
My patch moved the vmalloc() right after
kvm_mmu_slot_remove_write_access() and made this chance bigger:
because vmalloc() takes some time.

  Thanks,
    Takuya


> 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.
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