KVM: x86: Drop WARNs that assert a triple fault never "escapes" from L2

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2115238



commit 45846661
Author: Sean Christopherson <seanjc@google.com>
Date:   Thu Apr 7 00:23:13 2022 +0000

    KVM: x86: Drop WARNs that assert a triple fault never "escapes" from L2

    Remove WARNs that sanity check that KVM never lets a triple fault for L2
    escape and incorrectly end up in L1.  In normal operation, the sanity
    check is perfectly valid, but it incorrectly assumes that it's impossible
    for userspace to induce KVM_REQ_TRIPLE_FAULT without bouncing through
    KVM_RUN (which guarantees kvm_check_nested_state() will see and handle
    the triple fault).

    The WARN can currently be triggered if userspace injects a machine check
    while L2 is active and CR4.MCE=0.  And a future fix to allow save/restore
    of KVM_REQ_TRIPLE_FAULT, e.g. so that a synthesized triple fault isn't
    lost on migration, will make it trivially easy for userspace to trigger
    the WARN.

    Clearing KVM_REQ_TRIPLE_FAULT when forcibly leaving guest mode is
    tempting, but wrong, especially if/when the request is saved/restored,
    e.g. if userspace restores events (including a triple fault) and then
    restores nested state (which may forcibly leave guest mode).  Ignoring
    the fact that KVM doesn't currently provide the necessary APIs, it's
    userspace's responsibility to manage pending events during save/restore.

      ------------[ cut here ]------------
      WARNING: CPU: 7 PID: 1399 at arch/x86/kvm/vmx/nested.c:4522 nested_vmx_vmexit+0x7fe/0xd90 [kvm_intel]
      Modules linked in: kvm_intel kvm irqbypass
      CPU: 7 PID: 1399 Comm: state_test Not tainted 5.17.0-rc3+ #808
      Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
      RIP: 0010:nested_vmx_vmexit+0x7fe/0xd90 [kvm_intel]
      Call Trace:
       <TASK>
       vmx_leave_nested+0x30/0x40 [kvm_intel]
       vmx_set_nested_state+0xca/0x3e0 [kvm_intel]
       kvm_arch_vcpu_ioctl+0xf49/0x13e0 [kvm]
       kvm_vcpu_ioctl+0x4b9/0x660 [kvm]
       __x64_sys_ioctl+0x83/0xb0
       do_syscall_64+0x3b/0xc0
       entry_SYSCALL_64_after_hwframe+0x44/0xae
       </TASK>
      ---[ end trace 0000000000000000 ]---

    Fixes: cb6a32c2 ("KVM: x86: Handle triple fault in L2 without killing L1")
    Cc: stable@vger.kernel.org
    Cc: Chenyi Qiang <chenyi.qiang@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
    Message-Id: <20220407002315.78092-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>