KVM: Initialize debugfs_dentry when a VM is created to avoid NULL deref

KVM: Initialize debugfs_dentry when a VM is created to avoid NULL deref

Initialize debugfs_entry to its semi-magical -ENOENT value when the VM
is created.  KVM's teardown when VM creation fails is kludgy and calls
kvm_uevent_notify_change() and kvm_destroy_vm_debugfs() even if KVM never
attempted kvm_create_vm_debugfs().  Because debugfs_entry is zero
initialized, the IS_ERR() checks pass and KVM derefs a NULL pointer.

  BUG: kernel NULL pointer dereference, address: 0000000000000018
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 1068b1067 P4D 1068b1067 PUD 1068b0067 PMD 0
  Oops: 0000 [#1] SMP
  CPU: 0 PID: 871 Comm: repro Not tainted 5.18.0-rc1+ #825
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:__dentry_path+0x7b/0x130
  Call Trace:
   <TASK>
   dentry_path_raw+0x42/0x70
   kvm_uevent_notify_change.part.0+0x10c/0x200 [kvm]
   kvm_put_kvm+0x63/0x2b0 [kvm]
   kvm_dev_ioctl+0x43a/0x920 [kvm]
   __x64_sys_ioctl+0x83/0xb0
   do_syscall_64+0x31/0x50
   entry_SYSCALL_64_after_hwframe+0x44/0xae
   </TASK>
  Modules linked in: kvm_intel kvm irqbypass

Fixes: a44a4cc1c969 ("KVM: Don't create VM debugfs files outside of the VM directory")
Cc: stable@vger.kernel.org
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@google.com>
Reported-by: syzbot+df6fbbd2ee39f21289ef@syzkaller.appspotmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20220415004622.2207751-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>