UBUNTU-CVE-2026-68428: Use After Free
In the Linux kernel, the following vulnerability has been resolved: KVM: x86/mmu: Fix use-after-free on vendor module reload mmudestroycaches() destroys ptelistdesccache and mmupageheadercache, but leaves both pointers unchanged. The pointers live in kvm.ko, and therefore survive when a vendor module is unloaded while kvm.ko remains loaded. If creation of ptelistdesccache fails during a subsequent vendor module load, its assignment sets ptelistdesccache to NULL and the error path calls mmudestroycaches(). mmupageheadercache still points to the cache destroyed during the preceding vendor module unload. Passing that stale pointer to kmemcachedestroy() causes a slab use-after-free. Reproduce the issue on a v7.1.3 kernel with CONFIGKASAN=y, CONFIGKASANGENERIC=y, CONFIGKVM=m, and CONFIGKVMINTEL=m. A one-shot test hook forces ptelistdesccache to NULL on the second invocation of kvmmmuvendormoduleinit(): 1. Load kvm.ko and kvm-intel.ko, creating both caches. 2. Unload only kvmintel, leaving kvm.ko loaded. 3. Reload kvmintel and force initialization through the -ENOMEM path. KASAN reports: BUG: KASAN: slab-use-after-free in kvmmmuvendormoduleinit+0x5b/0x170 [kvm] ... kmemcachedestroy+0x21/0x1d0 kvmmmuvendormoduleinit+0x5b/0x170 [kvm] ... Allocated by task 16817: kmemcachecreateargs+0x12c/0x3b0 kmemcachecreate.constprop.0+0xb6/0xf0 [kvm] kvmmmuvendormoduleinit+0x13b/0x170 [kvm] ... Freed by task 16820: kmemcachedestroy+0x117/0x1d0 kvmmmuvendormoduleexit+0x21/0x30 [kvm] Clear both pointers immediately after destroying their caches so that the stored state reflects the caches' lifetime and repeated cleanup is safe. With the fix applied, the same injected vendor module reload fails with -ENOMEM as expected and produces no KASAN report.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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Clear both pte_list_desc_cache and mmu_page_header_cache pointers immediately after destroying their caches in mmu_destroy_caches(), so the stored state reflects the caches' lifetime and repeated cleanup is safe.
Event History
Frequently Asked Questions
Which systems are exposed to this issue?
Systems using the affected Ubuntu kernel packages are exposed when KVM is built and used as loadable modules, including kvm.ko and an x86 vendor module such as kvm-intel.ko. The described sequence requires kvm.ko to remain loaded while the vendor module is unloaded and later reloaded.
What must happen for the use-after-free to be triggered?
After both MMU caches are created, the vendor KVM module must be unloaded without unloading kvm.ko. On a later vendor-module load, creation of pte_list_desc_cache must fail, causing the initialization error path to destroy a stale mmu_page_header_cache pointer.
Is ordinary KVM module loading sufficient to trigger the problem?
No. The documented reproduction requires a vendor-module reload followed by an allocation failure during MMU cache initialization; the test uses a hook to force the -ENOMEM path on the second initialization.
How can an administrator identify a suspected occurrence?
On a kernel built with KASAN enabled, the failure is reported as a KASAN slab-use-after-free in kvm_mmu_vendor_module_init. The affected module state is kvm.ko remaining loaded after its vendor module, such as kvm_intel, has been unloaded.