linux-kernel - Re: [PATCH 2/2] KVM: MMU: fix relaxing permission

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Date:	Thu, 27 May 2010 19:00:50 +0800
From:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] KVM: MMU: fix relaxing permission



Avi Kivity wrote:
> On 05/26/2010 05:46 AM, Xiao Guangrong wrote:
>> There is a relaxing permission operation in set_spte():
>>
>> if guest's CR0.WP is not set and R/W #PF occurs in supervisor-level,
>> the mapping path might set to writable, then user can allow to write.
>>
>> @@ -1859,8 +1859,7 @@ static int set_spte(struct kvm_vcpu *vcpu, u64
>> *sptep,
>>
>>       spte |= (u64)pfn<<  PAGE_SHIFT;
>>
>> -    if ((pte_access&  ACC_WRITE_MASK)
>> -        || (write_fault&&  !is_write_protection(vcpu)&&  !user_fault)) {
>> +    if (pte_access&  ACC_WRITE_MASK) {
>>
>>    
> 
> The host always sets cr0.wp (in shadow mode) so we can write protect
> page tables.  So when the guest clears cr0.wp, we emulate a gpte with
> gpte.w=0 and gpte.u=1 in two ways:
> 
> - spte.w=1, spte.u=0: this will allow the guest kernel to write but trap
> on guest user access
> - spte.w=0, spte.u=1: allows guest user access but traps on guest kernel
> writes
> 
> If the guest attempts an access that is currently disallowed, we switch
> to the other spte encoding.

Avi,

Thanks for your explanation, but i not see where to implement what you say,
could you please point it out for me? :-(

And, i think use 'spte.w=1, spte.u=0' to emulate 'guest cr0.wp=0 and gpte.w=0'
is not a good way since it can completely stop user process access, but in this
case, user process is usually read and kernel lazily to write, just like vdso,
it will generate a lots of #PF
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