Re: [PATCH 1/4] KVM: VMX: Always clear vmx->fail on emulation_required

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2021-12-07 at 19:30 +0000, Sean Christopherson wrote:
> 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
>   (c) 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
> 
> Fixes: c8607e4a086f ("KVM: x86: nVMX: don't fail nested VM entry on invalid guest state if !from_vmentry")
> Reported-by: syzbot+f1d2136db9c80d4733e8@syzkaller.appspotmail.com
> Cc: Maxim Levitsky <mlevitsk@redhat.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index efcc5a58abbc..9e415e5a91ab 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6631,9 +6631,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	 * consistency check VM-Exit due to invalid guest state and bail.
>  	 */
>  	if (unlikely(vmx->emulation_required)) {
> -
> -		/* We don't emulate invalid state of a nested guest */
> -		vmx->fail = is_guest_mode(vcpu);
> +		vmx->fail = 0;
>  
>  		vmx->exit_reason.full = EXIT_REASON_INVALID_STATE;
>  		vmx->exit_reason.failed_vmentry = 1;

Now after swapping in all of the gory details, this does make sense.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Best regards,
	Maxim Levitsky