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Thu, 5 Mar 2026 07:59:03 +0000 Date: Thu, 5 Mar 2026 15:55:55 +0800 From: Yan Zhao To: Sean Christopherson CC: Paolo Bonzini , , , Rick Edgecombe , Yosry Ahmed Subject: Re: [PATCH] KVM: x86/mmu: Don't create SPTEs for addresses that aren't mappable Message-ID: Reply-To: Yan Zhao References: <20260219002241.2908563-1-seanjc@google.com> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <20260219002241.2908563-1-seanjc@google.com> X-ClientProxiedBy: SI1PR02CA0032.apcprd02.prod.outlook.com (2603:1096:4:1f6::12) To PH0PR11MB7472.namprd11.prod.outlook.com (2603:10b6:510:28c::12) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH0PR11MB7472:EE_|DS0PR11MB7768:EE_ X-MS-Office365-Filtering-Correlation-Id: 994a92e3-4ac6-49ee-b941-08de7a8d111c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|376014|366016|1800799024; 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Attempting to create an SPTE > can cause KVM to drop the unmappable bits, and thus install a bad SPTE. > E.g. when starting a walk, the TDP MMU will round the GFN based on the > root level, and drop the upper bits. > > Exit with -EFAULT in the unlikely scenario userspace is misbehaving and > created a memslot that can't be addressed, e.g. if userspace installed > memory above the guest.MAXPHYADDR defined in CPUID, as there's nothing KVM > can do to make forward progress, and there _is_ a memslot for the address. > For emulated MMIO, KVM can at least kick the bad address out to userspace > via a normal MMIO exit. > > The flaw has existed for a very long time, and was exposed by commit > 988da7820206 ("KVM: x86/tdp_mmu: WARN if PFN changes for spurious faults") > thanks to a syzkaller program that prefaults memory at GPA 0x1000000000000 > and then faults in memory at GPA 0x0 (the extra-large GPA gets wrapped to > '0'). If the scenario is: when ad bit is disabled, prefault memory at GPA 0x0, then guest reads memory at GPA 0x1000000000000, would fast_page_fault() fix a wrong wrapped sptep for GPA 0x1000000000000? Do we need to check fault->addr in fast_page_fault() as well? > WARNING: arch/x86/kvm/mmu/tdp_mmu.c:1183 at kvm_tdp_mmu_map+0x5c3/0xa30 [kvm], CPU#125: syz.5.22/18468 > CPU: 125 UID: 0 PID: 18468 Comm: syz.5.22 Tainted: G S W 6.19.0-smp--23879af241d6-next #57 NONE > Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN > Hardware name: Google Izumi-EMR/izumi, BIOS 0.20250917.0-0 09/17/2025 > RIP: 0010:kvm_tdp_mmu_map+0x5c3/0xa30 [kvm] > Call Trace: > > kvm_tdp_page_fault+0x107/0x140 [kvm] > kvm_mmu_do_page_fault+0x121/0x200 [kvm] > kvm_arch_vcpu_pre_fault_memory+0x18c/0x230 [kvm] > kvm_vcpu_pre_fault_memory+0x116/0x1e0 [kvm] > kvm_vcpu_ioctl+0x3a5/0x6b0 [kvm] > __se_sys_ioctl+0x6d/0xb0 > do_syscall_64+0x8d/0x900 > entry_SYSCALL_64_after_hwframe+0x4b/0x53 > > > In practice, the flaw is benign (other than the new WARN) as it only > affects guests that ignore guest.MAXPHYADDR (e.g. on CPUs with 52-bit > physical addresses but only 4-level paging) or guests being run by a > misbehaving userspace VMM (e.g. a VMM that ignored allow_smaller_maxphyaddr > or is pre-faulting bad addresses). > > For non-TDP shadow paging, always clear the unmappable mask as the flaw > only affects GPAs affected. For 32-bit paging, 64-bit virtual addresses > simply don't exist. Even when software can shove a 64-bit address > somewhere, e.g. into SYSENTER_EIP, the value is architecturally truncated > before it reaches the page table walker. And for 64-bit paging, KVM's use > of 4-level vs. 5-level paging is tied to the guest's CR4.LA57, i.e. KVM > won't observe a 57-bit virtual address with a 4-level MMU.