Re: [PATCH for 3.3] KVM: Fix write protection race during dirty logging

[Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>]

On 02/06/2012 11:46 AM, Takuya Yoshikawa wrote:

> (2012/02/06 12:40), Xiao Guangrong wrote:
>> On 02/05/2012 07:42 PM, Takuya Yoshikawa wrote:
>>
>>> From: Takuya Yoshikawa<yoshikawa.takuya@oss.ntt.co.jp>
>>>
>>> This patch fixes a race introduced by:
>>>
>>>    commit 95d4c16ce78cb6b7549a09159c409d52ddd18dae
>>>    KVM: Optimize dirty logging by rmap_write_protect()
>>>
>>> During protecting pages for dirty logging, other threads may also try
>>> to protect a page in mmu_sync_children() or kvm_mmu_get_page().
>>>
>>> In such a case, because get_dirty_log releases mmu_lock before flushing
>>> TLB's, the following race condition can happen:
>>>
>>>    A (get_dirty_log)     B (another thread)
>>>
>>>    lock(mmu_lock)
>>>    clear pte.w
>>>    unlock(mmu_lock)
>>>                          lock(mmu_lock)
>>>                          pte.w is already cleared
>>>                          unlock(mmu_lock)
>>>                          skip TLB flush
>>>                          return
>>>    ...
>>>    TLB flush
>>>
>>> Though thread B assumes the page has already been protected when it
>>> returns, the remaining TLB entry will break that assumption.
>>>
>>> This patch fixes this problem by making get_dirty_log hold the mmu_lock
>>> until it flushes the TLB's.
>>>
>>
>>
>> I do not think this is a problem since the dirty page is logged when
>> the writeable spte is being set, and in the end of get_dirty_log, all
>> TLBs are always flushed.
>>
> 
> The victim is not GET_DIRTY_LOG but thread B;  it needs to assure the page
> is protected before returning.
> 


Ah, right!

Reviewed-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>