CVE-2026-74516: KVM: SVM: Update x2APIC MSR intercepts if AVIC is inhibited while L2 is active

In the Linux kernel, the following vulnerability has been resolved:

KVM: SVM: Update x2APIC MSR intercepts if AVIC is inhibited while L2 is active

Always update x2APIC MSR intercepts for L1 when AVIC is deactivated, even if L2 is active and KVM is using a separate MSR bitmap to run L2. If AVIC is fully enabled prior to running L2, and is then inhibited while L2 is active (for a VM-scoped inhibit), then KVM will run L1 with AVIC disabled, but with x2APIC MSR intercepts disabled, i.e. will allow L1 to read most of the host's APIC state, send arbitrary interrupts, change task priority, and ultimately trivially DoS the host.

E.g. sending a self-IPI in L1 on HYPERVREENLIGHTENMENTVECTOR, 0xee, with CONFIGHYPERV=n in the host kernel as a "safe" PoC, yields:

Spurious interrupt (vector 0xee) on CPU#425. Acked

And hacking KVM to abuse kvmsetpostedintrwakeuphandler() to register a handler and WARN on POSTEDINTRWAKEUPVECTOR yields:

------------[ cut here ]------------ WARNING: arch/x86/kvm/svm/svm.c:5594 at piwakeuphandler+0x9/0x10 [kvmamd], CPU#156: nestedx2apict/316940 CPU: 156 UID: 0 PID: 316940 Comm: nestedx2apict Tainted: G S U Tainted: [S]=CPUOUTOFSPEC, [U]=USER Hardware name: Google Astoria-Turin/astoria, BIOS 0.20260209.0-0 02/09/2026 RIP: 0010:piwakeuphandler+0x9/0x10 [kvmamd] Call Trace: <IRQ> sysveckvmpostedintrwakeupipi+0x64/0x80 </IRQ> <TASK> asmsysveckvmpostedintrwakeupipi+0x1a/0x20 RIP: 0010:vcpurun+0x1430/0x1e40 [kvm] kvmarchvcpuioctlrun+0x2c1/0x600 [kvm] kvmvcpuioctl+0x580/0x6b0 [kvm] sesysioctl+0x6d/0xb0 dosyscall64+0x10a/0x480 entrySYSCALL64afterhwframe+0x4b/0x53 RIP: 0033:0x46ff4b </TASK> ---[ end trace 0000000000000000 ]---