CVE-2026-99176: KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry
In the Linux kernel, the following vulnerability has been resolved:
KVM: nVMX: Reset MMU back to the root MMU if loading L2 state fails for !VM-Entry
Reset the vCPU's MMU pointer back to the root MMU if loading L2 state fails in the "not from VM-Entry" case, i.e. when emulating RSM or handling KVMSETNESTEDSTATE. Failure to unwind the MMU pointer results in KVM trying to run L1 with a nested MMU, and is especially problematic if the vCPU manages to free its nested state before the VM crashes. Constructing the root for a nested EPT MMU goes through vmcs12, which is dynamically allocated/freed when nVMX is enabled/disabled. And so if the nested state is freed with the MMU still pointing at the nested MMU, KVM will hit a NULL pointer deref when attempting to get L1=>L2 EPTP.
BUG: kernel NULL pointer dereference, address: 0000000000000078 #PF: supervisor read access in kernel mode #PF: errorcode(0x0000) - not-present page PGD 114a2d067 P4D 111743067 PUD 11141f067 PMD 0 Oops: Oops: 0000 [#1] SMP CPU: 20 UID: 1000 PID: 1272 Comm: nestedtdpfaul Not tainted 7.3.0-rc4-8c89f11d1a99b-sink-vm #1914 PREEMPTLAZY Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015 RIP: 0010:nestedeptgeteptp+0xb/0x10 [kvmintel] Call Trace: <TASK> kvmmmuload+0xcd/0x870 [kvm] kvmarchvcpuioctlrun+0x18f5/0x1f80 [kvm] kvmvcpuioctl+0x2c7/0x970 [kvm] x64sysioctl+0x90/0xd0 dosyscall64+0x9c/0x4d0 entrySYSCALL64afterhwframe+0x4b/0x53 RIP: 0033:0x47491b </TASK> Modules linked in: kvmintel kvm irqbypass CR2: 0000000000000078 ---[ end trace 0000000000000000 ]--- RIP: 0010:nestedeptgeteptp+0xb/0x10 [kvmintel]
[sean: write changelog]
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Compensating control
In KVM nVMX, reset the vCPU's MMU pointer back to the root MMU when loading L2 state fails outside the VM-Entry case, preventing access through a freed nested MMU.