CVE-2021-47092

In the Linux kernel, the following vulnerability has been resolved: KVM:
VMX: Always clear vmx->fail on emulation_required Revert a relatively
recent change that set vmx->fail if the vCPU is in L2 and
emulation_required is true, as that behavior is completely bogus. Setting
vmx->fail and synthesizing a VM-Exit is contradictory and wrong: (a) it’s
impossible to have both a VM-Fail and VM-Exit (b) vmcs.EXIT_REASON is not
modified on VM-Fail © emulation_required refers to guest state and guest
state checks are always VM-Exits, not VM-Fails. For KVM specifically,
emulation_required is handled before nested exits in __vmx_handle_exit(),
thus setting vmx->fail has no immediate effect, i.e. KVM calls into
handle_invalid_guest_state() and vmx->fail is ignored. Setting vmx->fail
can ultimately result in a WARN in nested_vmx_vmexit() firing when tearing
down the VM as KVM never expects vmx->fail to be set when L2 is active, KVM
always reflects those errors into L1. ------------[ cut here ]------------
WARNING: CPU: 0 PID: 21158 at arch/x86/kvm/vmx/nested.c:4548
nested_vmx_vmexit+0x16bd/0x17e0 arch/x86/kvm/vmx/nested.c:4547 Modules
linked in: CPU: 0 PID: 21158 Comm: syz-executor.1 Not tainted
5.16.0-rc3-syzkaller #0 Hardware name: Google Google Compute Engine/Google
Compute Engine, BIOS Google 01/01/2011 RIP:
0010:nested_vmx_vmexit+0x16bd/0x17e0 arch/x86/kvm/vmx/nested.c:4547 Code:
<0f> 0b e9 2e f8 ff ff e8 57 b3 5d 00 0f 0b e9 00 f1 ff ff 89 e9 80 Call
Trace: vmx_leave_nested arch/x86/kvm/vmx/nested.c:6220 [inline]
nested_vmx_free_vcpu+0x83/0xc0 arch/x86/kvm/vmx/nested.c:330
vmx_free_vcpu+0x11f/0x2a0 arch/x86/kvm/vmx/vmx.c:6799
kvm_arch_vcpu_destroy+0x6b/0x240 arch/x86/kvm/x86.c:10989
kvm_vcpu_destroy+0x29/0x90 arch/x86/kvm/…/…/…/virt/kvm/kvm_main.c:441
kvm_free_vcpus arch/x86/kvm/x86.c:11426 [inline]
kvm_arch_destroy_vm+0x3ef/0x6b0 arch/x86/kvm/x86.c:11545 kvm_destroy_vm
arch/x86/kvm/…/…/…/virt/kvm/kvm_main.c:1189 [inline]
kvm_put_kvm+0x751/0xe40 arch/x86/kvm/…/…/…/virt/kvm/kvm_main.c:1220
kvm_vcpu_release+0x53/0x60 arch/x86/kvm/…/…/…/virt/kvm/kvm_main.c:3489
__fput+0x3fc/0x870 fs/file_table.c:280 task_work_run+0x146/0x1c0
kernel/task_work.c:164 exit_task_work include/linux/task_work.h:32 [inline]
do_exit+0x705/0x24f0 kernel/exit.c:832 do_group_exit+0x168/0x2d0
kernel/exit.c:929 get_signal+0x1740/0x2120 kernel/signal.c:2852
arch_do_signal_or_restart+0x9c/0x730 arch/x86/kernel/signal.c:868
handle_signal_work kernel/entry/common.c:148 [inline]
exit_to_user_mode_loop kernel/entry/common.c:172 [inline]
exit_to_user_mode_prepare+0x191/0x220 kernel/entry/common.c:207
__syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
syscall_exit_to_user_mode+0x2e/0x70 kernel/entry/common.c:300
do_syscall_64+0x53/0xd0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x44/0xae