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[35.230.65.123]) by smtp.gmail.com with ESMTPSA id mi14-20020a17090b4b4e00b001f2184008c7sm9524pjb.53.2022.07.19.16.21.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 Jul 2022 16:21:37 -0700 (PDT) Date: Tue, 19 Jul 2022 23:21:33 +0000 From: Sean Christopherson To: Lai Jiangshan Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Paolo Bonzini , Vitaly Kuznetsov , Maxim Levitsky , David Matlack , Lai Jiangshan Subject: Re: [PATCH V3 09/12] KVM: X86/MMU: Move the verifying of NPT's PDPTE in FNAME(fetch) Message-ID: References: <20220521131700.3661-1-jiangshanlai@gmail.com> <20220521131700.3661-10-jiangshanlai@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220521131700.3661-10-jiangshanlai@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 21, 2022, Lai Jiangshan wrote: > From: Lai Jiangshan > > FNAME(page_fault) verifies PDPTE for nested NPT in PAE paging mode > because nested_svm_get_tdp_pdptr() reads the guest NPT's PDPTE from > memory unconditionally for each call. > > The verifying is complicated and it works only when mmu->pae_root > is always used when the guest is PAE paging. Why is this relevant? It's not _that_ complicated, and even if it were, I don't see how calling that out helps the reader understand the motivation for this patch. > Move the verifying code in FNAME(fetch) and simplify it since the local > shadow page is used and it can be walked in FNAME(fetch) and unlinked > from children via drop_spte(). > > It also allows for mmu->pae_root NOT to be used when it is NOT required Avoid leading with pronous, "it" is ambiguous, e.g. at first I thought "it' meant moving the code, but what "it" really means is using the iterator from the shadow page walk instead of hardcoding a pae_root lookup. And changing from pae_root to it.sptep needs to be explicitly called out. It's a subtle but important detail. And if you call that out, then it's more obvious why this patch is relevant to not having to use pae_root for a 64-bit host with NPT. > to be put in a 32bit CR3. > > Signed-off-by: Lai Jiangshan > --- > arch/x86/kvm/mmu/paging_tmpl.h | 72 ++++++++++++++++------------------ > 1 file changed, 33 insertions(+), 39 deletions(-) > > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h > index cd6032e1947c..67c419bce1e5 100644 > --- a/arch/x86/kvm/mmu/paging_tmpl.h > +++ b/arch/x86/kvm/mmu/paging_tmpl.h > @@ -659,6 +659,39 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, > clear_sp_write_flooding_count(it.sptep); > drop_large_spte(vcpu, it.sptep); > > + /* > + * When nested NPT enabled and L1 is PAE paging, > + * mmu->get_pdptrs() which is nested_svm_get_tdp_pdptr() reads > + * the guest NPT's PDPTE from memory unconditionally for each > + * call. > + * > + * The guest PAE root page is not write-protected. > + * > + * The mmu->get_pdptrs() in FNAME(walk_addr_generic) might get > + * a value different from previous calls or different from the > + * return value of mmu->get_pdptrs() in mmu_alloc_shadow_roots(). > + * > + * It will cause the following code installs the spte in a wrong > + * sp or links a sp to a wrong parent if the return value of > + * mmu->get_pdptrs() is not verified unchanged since > + * FNAME(gpte_changed) can't check this kind of change. > + * > + * Verify the return value of mmu->get_pdptrs() (only the gfn > + * in it needs to be checked) and drop the spte if the gfn isn't > + * matched. > + * > + * Do the verifying unconditionally when the guest is PAE > + * paging no matter whether it is nested NPT or not to avoid > + * complicated code. > + */ > + if (vcpu->arch.mmu->cpu_role.base.level == PT32E_ROOT_LEVEL && > + it.level == PT32E_ROOT_LEVEL && > + is_shadow_present_pte(*it.sptep)) { > + sp = to_shadow_page(*it.sptep & PT64_BASE_ADDR_MASK); For this patch, it's probably worth a WARN_ON_ONCE(sp->spt != vcpu->arch.mmu->pae_root); Mostly so that when the future patch stops using pae_root for 64-bit NPT hosts, there's a code change for this particular logic that is very much relevant to that change. > + if (gw->table_gfn[it.level - 2] != sp->gfn) > + drop_spte(vcpu->kvm, it.sptep); > + }