KVM: Prevent module exit until all VMs are freed
Tie the lifetime the KVM module to the lifetime of each VM via kvm.users_count. This way anything that grabs a reference to the VM via kvm_get_kvm() cannot accidentally outlive the KVM module. Prior to this commit, the lifetime of the KVM module was tied to the lifetime of /dev/kvm file descriptors, VM file descriptors, and vCPU file descriptors by their respective file_operations "owner" field. This approach is insufficient because references grabbed via kvm_get_kvm() do not prevent closing any of the aforementioned file descriptors. This fixes a long standing theoretical bug in KVM that at least affects async page faults. kvm_setup_async_pf() grabs a reference via kvm_get_kvm(), and drops it in an asynchronous work callback. Nothing prevents the VM file descriptor from being closed and the KVM module from being unloaded before this callback runs. Fixes: af585b92 ("KVM: Halt vcpu if page it tries to access is swapped out") Fixes: 3d3aab1b ("KVM: set owner of cpu and vm file operations") Cc: stable@vger.kernel.org Suggested-by:Ben Gardon <bgardon@google.com> [ Based on a patch from Ben implemented for Google's kernel. ] Signed-off-by: