CVE-2026-64247 in Linux
Summary
by MITRE • 07/24/2026
In the Linux kernel, the following vulnerability has been resolved:
KVM: x86: hyper-v: Bound the bank index when querying sparse banks
When checking if a VP ID is included in a sparse bank set, explicitly check that the ID can actually be contained in a sparse bank (the TLFS allows for a maximum of 64 banks of 64 vCPUs each). When handling a paravirtual TLB flush for L2, the VP ID is copied verbatim from the enlightened VMCS, without any bounds check, i.e. isn't guaranteed to be under the limit of 4096.
Failure to check the bounds of the VP ID leads to an out-of-bounds read when testing the sparse bank, and super strictly speaking could lead to KVM performing an unnecessary TLB flush for an L2 vCPU.
================================================================== BUG: KASAN: use-after-free in hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
Read of size 8 at addr ffff88811ba5f598 by task hyperv_evmcs/2802
CPU: 12 UID: 1000 PID: 2802 Comm: hyperv_evmcs Not tainted 7.1.0-rc2 #7 PREEMPT Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 Call Trace: <TASK> dump_stack_lvl+0x51/0x60 print_report+0xcb/0x5d0 kasan_report+0xb4/0xe0 kasan_check_range+0x35/0x1b0 hv_is_vp_in_sparse_set+0x85/0x100 [kvm]
kvm_hv_flush_tlb+0xe9e/0x16c0 [kvm]
kvm_hv_hypercall+0xe6b/0x1e60 [kvm]
vmx_handle_exit+0x485/0x1b60 [kvm_intel]
kvm_arch_vcpu_ioctl_run+0x22e3/0x5070 [kvm]
kvm_vcpu_ioctl+0x5d0/0x10c0 [kvm]
__x64_sys_ioctl+0x129/0x1a0 do_syscall_64+0xb9/0xcf0 entry_SYSCALL_64_after_hwframe+0x4b/0x53 RIP: 0033:0x7f0e62d1a9bf </TASK>
The buggy address belongs to the physical page: page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffffffffffffffff pfn:0x11ba5f flags: 0x4000000000000000(zone=1) raw: 4000000000000000 0000000000000000 00000000ffffffff 0000000000000000 raw: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000000 page dumped because: kasan: bad access detected
Memory state around the buggy address: ffff88811ba5f480: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88811ba5f500: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff >ffff88811ba5f580: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ^ ffff88811ba5f600: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ffff88811ba5f680: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ================================================================== Disabling lock debugging due to kernel taint
Opportunistically add a compile time assertion to ensure the maximum number of sparse banks exactly matches the number of possible bits in the passed in mask.
[sean: add KASAN splat, drop comment, add assert, massage changelog]
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 07/24/2026
The vulnerability resides within the Linux kernel's KVM subsystem, specifically affecting the x86 hypervisor implementation when handling Hyper-V paravirtualized TLB flush operations. This issue manifests in the hv_is_vp_in_sparse_set function where a bounds check is missing for virtual processor IDs during sparse bank queries. The flaw originates from copying VP IDs verbatim from the enlightened VMCS without validating that they fall within acceptable limits, potentially exceeding the maximum of 4096 vCPUs (64 banks of 64 vCPUs each as defined by the Hyper-V TLFS specification). This oversight creates a potential out-of-bounds read condition when testing sparse bank membership, which can trigger a use-after-free error detected by KASAN memory safety framework. The vulnerability is classified under CWE-129 as an insufficient input validation and relates to improper bounds checking in kernel memory operations.
The technical execution of this vulnerability occurs during nested virtualization scenarios where L2 guest VMs perform TLB flush operations through Hyper-V hypercalls. When processing these calls, the system directly copies VP IDs from the VMCS without validating against the maximum allowed bank index, leading to memory access violations at addresses such as ffff88811ba5f598. The KASAN report reveals a read operation of size 8 bytes occurring at an invalid memory location, indicating that the system attempts to access memory beyond the allocated sparse bank structure boundaries. This misbehavior can result in either immediate kernel crashes or more subtle issues like unnecessary TLB flushes for L2 vCPUs, impacting performance and potentially creating security exposure vectors. The attack surface is primarily within hypervisor code paths handling nested virtualization with Hyper-V enlightened VMCS implementations.
The operational impact of this vulnerability extends beyond simple memory corruption, as it affects the stability and reliability of nested virtualization environments. Systems utilizing KVM with Hyper-V paravirtualized guests may experience kernel panics or unexpected behavior during TLB flush operations, particularly when large numbers of vCPUs are involved. The vulnerability affects the integrity of the hypervisor's sparse bank management logic, which is critical for maintaining efficient memory management in virtualized environments. Additionally, this flaw may enable potential privilege escalation paths or information disclosure if attackers can manipulate VP ID values to trigger the out-of-bounds access conditions. The issue aligns with ATT&CK technique T1059.003 (Command and Scripting Interpreter: Windows Command Shell) when exploited through malicious VM configuration, and could support broader hypervisor escape techniques.
Mitigation strategies for this vulnerability include implementing explicit bounds checking for VP IDs before processing sparse bank queries, ensuring that all input values fall within the defined maximum limits of 4096 vCPUs. The fix incorporates compile-time assertions to validate that the maximum number of sparse banks matches the bit mask capacity, preventing configuration mismatches that could lead to similar issues. System administrators should ensure their kernel versions include the patched KVM implementation and monitor for potential instability in nested virtualization environments. Additionally, implementing proper input validation mechanisms and memory access controls can prevent similar issues in other hypervisor components. Regular kernel updates and security audits of virtualization infrastructure are essential practices to maintain system integrity and protect against such memory safety vulnerabilities that could be exploited by malicious actors targeting virtualized environments.