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Received-SPF: Pass (protection.outlook.com: domain of amd.com designates 165.204.84.17 as permitted sender) receiver=protection.outlook.com; client-ip=165.204.84.17; helo=SATLEXMB04.amd.com; pr=C Received: from SATLEXMB04.amd.com (165.204.84.17) by CH2PEPF0000014A.mail.protection.outlook.com (10.167.244.107) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.20.7784.11 via Frontend Transport; Wed, 17 Jul 2024 21:55:00 +0000 Received: from localhost (10.180.168.240) by SATLEXMB04.amd.com (10.181.40.145) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.39; Wed, 17 Jul 2024 16:54:59 -0500 Date: Wed, 17 Jul 2024 16:53:35 -0500 From: Michael Roth To: Paolo Bonzini CC: , , Subject: Re: [PATCH 09/12] KVM: guest_memfd: move check for already-populated page to common code Message-ID: <20240717215335.spgo572mhpfqcq64@amd.com> References: <20240711222755.57476-1-pbonzini@redhat.com> <20240711222755.57476-10-pbonzini@redhat.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; 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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 17 Jul 2024 21:55:00.1642 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: ae83944e-57f7-43f6-301f-08dca6ab1b03 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[SATLEXMB04.amd.com] X-MS-Exchange-CrossTenant-AuthSource: CH2PEPF0000014A.namprd02.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY5PR12MB6323 On Thu, Jul 11, 2024 at 06:27:52PM -0400, Paolo Bonzini wrote: > Do not allow populating the same page twice with startup data. In the > case of SEV-SNP, for example, the firmware does not allow it anyway, > since the launch-update operation is only possible on pages that are > still shared in the RMP. > > Even if it worked, kvm_gmem_populate()'s callback is meant to have side > effects such as updating launch measurements, and updating the same > page twice is unlikely to have the desired results. > > Races between calls to the ioctl are not possible because kvm_gmem_populate() > holds slots_lock and the VM should not be running. But again, even if > this worked on other confidential computing technology, it doesn't matter > to guest_memfd.c whether this is an intentional attempt to do something > fishy, or missing synchronization in userspace, or even something > intentional. One of the racers wins, and the page is initialized by I think one of those "intentional"s was meant to be an "unintentional". > either kvm_gmem_prepare_folio() or kvm_gmem_populate(). > > Anyway, out of paranoia, adjust sev_gmem_post_populate() anyway to use > the same errno that kvm_gmem_populate() is using. > > Signed-off-by: Paolo Bonzini Assuming based on the discussion here that there will be some logic added to enforce that KVM_PRE_FAULT_MEMORY can only happen after finalization, I think the new checks make sense. Reviewed-by: Michael Roth > --- > arch/x86/kvm/svm/sev.c | 2 +- > virt/kvm/guest_memfd.c | 7 +++++++ > 2 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > index df8818759698..397ef9e70182 100644 > --- a/arch/x86/kvm/svm/sev.c > +++ b/arch/x86/kvm/svm/sev.c > @@ -2213,7 +2213,7 @@ static int sev_gmem_post_populate(struct kvm *kvm, gfn_t gfn_start, kvm_pfn_t pf > if (ret || assigned) { > pr_debug("%s: Failed to ensure GFN 0x%llx RMP entry is initial shared state, ret: %d assigned: %d\n", > __func__, gfn, ret, assigned); > - ret = -EINVAL; > + ret = ret ? -EINVAL : -EEXIST; > goto err; > } > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 509360eefea5..266810bb91c9 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -650,6 +650,13 @@ long kvm_gmem_populate(struct kvm *kvm, gfn_t start_gfn, void __user *src, long > break; > } > > + if (folio_test_uptodate(folio)) { > + folio_unlock(folio); > + folio_put(folio); > + ret = -EEXIST; > + break; > + } > + > folio_unlock(folio); > if (!IS_ALIGNED(gfn, (1 << max_order)) || > (npages - i) < (1 << max_order)) > -- > 2.43.0 > > >