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

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Date:	Thu, 27 May 2010 13:10:27 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.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

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.

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

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