CVE-2025-37957: KVM: SVM: Forcibly leave SMM mode on SHUTDOWN interception

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

KVM: SVM: Forcibly leave SMM mode on SHUTDOWN interception

Previously, commit ed129ec9057f ("KVM: x86: forcibly leave nested mode on vCPU reset") addressed an issue where a triple fault occurring in nested mode could lead to use-after-free scenarios. However, the commit did not handle the analogous situation for System Management Mode (SMM).

This omission results in triggering a WARN when KVM forces a vCPU INIT after SHUTDOWN interception while the vCPU is in SMM. This situation was reprodused using Syzkaller by:

1) Creating a KVM VM and vCPU 2) Sending a KVMSMI ioctl to explicitly enter SMM 3) Executing invalid instructions causing consecutive exceptions and eventually a triple fault

The issue manifests as follows:

WARNING: CPU: 0 PID: 25506 at arch/x86/kvm/x86.c:12112 kvmvcpureset+0x1d2/0x1530 arch/x86/kvm/x86.c:12112 Modules linked in: CPU: 0 PID: 25506 Comm: syz-executor.0 Not tainted 6.1.130-syzkaller-00157-g164fe5dde9b6 #0 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 RIP: 0010:kvmvcpureset+0x1d2/0x1530 arch/x86/kvm/x86.c:12112 Call Trace: <TASK> shutdowninterception+0x66/0xb0 arch/x86/kvm/svm/svm.c:2136 svminvokeexithandler+0x110/0x530 arch/x86/kvm/svm/svm.c:3395 svmhandleexit+0x424/0x920 arch/x86/kvm/svm/svm.c:3457 vcpuenterguest arch/x86/kvm/x86.c:10959 [inline] vcpurun+0x2c43/0x5a90 arch/x86/kvm/x86.c:11062 kvmarchvcpuioctlrun+0x50f/0x1cf0 arch/x86/kvm/x86.c:11283 kvmvcpuioctl+0x570/0xf00 arch/x86/kvm/../../../virt/kvm/kvmmain.c:4122 vfsioctl fs/ioctl.c:51 [inline] dosysioctl fs/ioctl.c:870 [inline] sesysioctl fs/ioctl.c:856 [inline] x64sysioctl+0x19a/0x210 fs/ioctl.c:856 dosyscallx64 arch/x86/entry/common.c:51 [inline] dosyscall64+0x35/0x80 arch/x86/entry/common.c:81 entrySYSCALL64afterhwframe+0x6e/0xd8

Architecturally, INIT is blocked when the CPU is in SMM, hence KVM's WARN() in kvmvcpureset() to guard against KVM bugs, e.g. to detect improper emulation of INIT. SHUTDOWN on SVM is a weird edge case where KVM needs to do something sane with the VMCB, since it's technically undefined, and INIT is the least awful choice given KVM's ABI.

So, double down on stuffing INIT on SHUTDOWN, and force the vCPU out of SMM to avoid any weirdness (and the WARN).

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

[sean: massage changelog, make it clear this isn't architectural behavior]