From: Xiao Guangrong Subject: Re: [PATCH 2/2] KVM: MMU: fix relaxing permission Date: Thu, 27 May 2010 19:00:50 +0800 Message-ID: <4BFE50E2.60203@cn.fujitsu.com> References: <4BFC8B0D.2060106@cn.fujitsu.com> <4BFC8B8C.7010402@cn.fujitsu.com> <4BFE4513.8090606@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM list To: Avi Kivity Return-path: In-Reply-To: <4BFE4513.8090606@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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