From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH v2 3/10] KVM: MMU: fix direct sp's access corruptted Date: Tue, 29 Jun 2010 10:38:50 +0300 Message-ID: <4C29A30A.8020107@redhat.com> References: <4C2498EC.2010006@cn.fujitsu.com> <4C249BAD.6000609@cn.fujitsu.com> <4C287081.40300@redhat.com> <4C287332.5080803@cn.fujitsu.com> <4C2883D3.2050606@redhat.com> <4C2949A5.1070303@cn.fujitsu.com> <4C299B7E.5020303@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , LKML , KVM list To: Xiao Guangrong Return-path: In-Reply-To: <4C299B7E.5020303@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 06/29/2010 10:06 AM, Avi Kivity wrote: > On 06/29/2010 04:17 AM, Xiao Guangrong wrote: >> >>> If B is writeable-and-dirty, then it's D bit is already set, and we >>> don't need to do anything. >>> >>> If B is writeable-and-clean, then we'll have an spte pointing to a >>> read-only sp, so we'll get a write fault on access and an >>> opportunity to >>> set the D bit. >>> >> Sorry, a typo in my reply, i mean mapping A and B both are >> writable-and-clean, >> while A occurs write-#PF, we should change A's spte map to writable >> sp, if we >> only update the spte in writable-and-clean sp(form readonly to >> writable), the B's >> D bit will miss set. > > Right. > > We need to update something to notice this: > > - FNAME(fetch)() to replace the spte > - FNAME(walk_addr)() to invalidate the spte > > I think FNAME(walk_addr) is the right place, we're updating the gpte, > so we should update the spte at the same time, just like a guest > write. But that will be expensive (there could be many sptes, so we > have to call kvm_mmu_pte_write()), so perhaps FNAME(fetch) is easier. > > We have now > > if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) > continue; > > So we need to add a check, if sp->role.access doesn't match pt_access > & pte_access, we need to get a new sp with the correct access (can > only change read->write). Note: - modifying walk_addr() to call kvm_mmu_pte_write() is probably not so bad. It's rare that a large pte walk sets the dirty bit, and it's probably rare to share those large ptes. Still, I think the fetch() change is better since it's more local. - there was once talk that instead of folding pt_access and pte_access together into the leaf sp->role.access, each sp level would have its own access permissions. In this case we don't even have to get a new direct sp, only change the PT_DIRECTORY_LEVEL spte to add write permissions (all direct sp's would be writeable and permissions would be controlled at their parent_pte level). Of course that's a much bigger change than this bug fix. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.