From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B2259C433ED for ; Thu, 20 May 2021 17:27:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 87FE56108C for ; Thu, 20 May 2021 17:27:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233620AbhETR2p (ORCPT ); Thu, 20 May 2021 13:28:45 -0400 Received: from mail.kernel.org ([198.145.29.99]:44120 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231680AbhETR2n (ORCPT ); Thu, 20 May 2021 13:28:43 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 75DE860240; Thu, 20 May 2021 17:27:18 +0000 (UTC) Date: Thu, 20 May 2021 18:27:16 +0100 From: Catalin Marinas To: Steven Price Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones Subject: Re: [PATCH v12 7/8] KVM: arm64: ioctl to fetch/store tags in a guest Message-ID: <20210520172713.GF12251@arm.com> References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-8-steven.price@arm.com> <20210520120556.GC12251@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 20, 2021 at 04:58:01PM +0100, Steven Price wrote: > On 20/05/2021 13:05, Catalin Marinas wrote: > > On Mon, May 17, 2021 at 01:32:38PM +0100, Steven Price wrote: > >> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > >> index e89a5e275e25..4b6c83beb75d 100644 > >> --- a/arch/arm64/kvm/arm.c > >> +++ b/arch/arm64/kvm/arm.c > >> @@ -1309,6 +1309,65 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm, > >> } > >> } > >> > >> +static int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm, > >> + struct kvm_arm_copy_mte_tags *copy_tags) > >> +{ > >> + gpa_t guest_ipa = copy_tags->guest_ipa; > >> + size_t length = copy_tags->length; > >> + void __user *tags = copy_tags->addr; > >> + gpa_t gfn; > >> + bool write = !(copy_tags->flags & KVM_ARM_TAGS_FROM_GUEST); > >> + int ret = 0; > >> + > >> + if (copy_tags->reserved[0] || copy_tags->reserved[1]) > >> + return -EINVAL; > >> + > >> + if (copy_tags->flags & ~KVM_ARM_TAGS_FROM_GUEST) > >> + return -EINVAL; > >> + > >> + if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK) > >> + return -EINVAL; > >> + > >> + gfn = gpa_to_gfn(guest_ipa); > >> + > >> + mutex_lock(&kvm->slots_lock); > >> + > >> + while (length > 0) { > >> + kvm_pfn_t pfn = gfn_to_pfn_prot(kvm, gfn, write, NULL); > >> + void *maddr; > >> + unsigned long num_tags = PAGE_SIZE / MTE_GRANULE_SIZE; > >> + > >> + if (is_error_noslot_pfn(pfn)) { > >> + ret = -EFAULT; > >> + goto out; > >> + } > >> + > >> + maddr = page_address(pfn_to_page(pfn)); > >> + > >> + if (!write) { > >> + num_tags = mte_copy_tags_to_user(tags, maddr, num_tags); > >> + kvm_release_pfn_clean(pfn); > > > > Do we need to check if PG_mte_tagged is set? If the page was not faulted > > into the guest address space but the VMM has the page, does the > > gfn_to_pfn_prot() guarantee that a kvm_set_spte_gfn() was called? If > > not, this may read stale tags. > > Ah, I hadn't thought about that... No I don't believe gfn_to_pfn_prot() > will fault it into the guest. It doesn't indeed. What it does is a get_user_pages() but it's not of much help since the VMM pte wouldn't be tagged (we would have solved lots of problems if we required PROT_MTE in the VMM...) > >> + } else { > >> + num_tags = mte_copy_tags_from_user(maddr, tags, > >> + num_tags); > >> + kvm_release_pfn_dirty(pfn); > >> + } > > > > Same question here, if the we can't guarantee the stage 2 pte being set, > > we'd need to set PG_mte_tagged. > > This is arguably worse as we'll be writing tags into the guest but > without setting PG_mte_tagged - so they'll be lost when the guest then > faults the pages in. Which sounds like it should break migration. > > I think the below should be safe, and avoids the overhead of setting the > flag just for reads. > > Thanks, > > Steve > > ----8<---- > page = pfn_to_page(pfn); > maddr = page_address(page); > > if (!write) { > if (test_bit(PG_mte_tagged, &page->flags)) > num_tags = mte_copy_tags_to_user(tags, maddr, > MTE_GRANULES_PER_PAGE); > else > /* No tags in memory, so write zeros */ > num_tags = MTE_GRANULES_PER_PAGE - > clear_user(tag, MTE_GRANULES_PER_PAGE); > kvm_release_pfn_clean(pfn); For ptrace we return a -EOPNOTSUPP if the address doesn't have VM_MTE but I don't think it makes sense here, so I'm fine with clearing the destination and assuming that the tags are zero (as they'd be on faulting into the guest. Another thing I forgot to ask, what's guaranteeing that the page supports tags? Does this ioctl ensure that it would attempt the tag copying from some device mapping? Do we need some kvm_is_device_pfn() check? I guess ZONE_DEVICE memory we just refuse to map in an earlier patch. > } else { > num_tags = mte_copy_tags_from_user(maddr, tags, > MTE_GRANULES_PER_PAGE); > kvm_release_pfn_dirty(pfn); > } > > if (num_tags != MTE_GRANULES_PER_PAGE) { > ret = -EFAULT; > goto out; > } > > if (write) > test_and_set_bit(PG_mte_tagged, &page->flags); I think a set_bit() would do, I doubt it's any more efficient. But why not add it in the 'else' block above where we actually wrote the tags? The copy function may have failed part-way through. Maybe your logic is correct though, there are invalid tags in the page. Just add a comment. -- Catalin