On Tue, Mar 8, 2022 at 1:40 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Thu, Mar 03, 2022, David Matlack wrote: > > 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: af585b921e5d ("KVM: Halt vcpu if page it tries to access is swapped out") > > And (or) > > Fixes: 3d3aab1b973b ("KVM: set owner of cpu and vm file operations") > > because the above is x86-centric, at a glance PPC and maybe s390 have issues > beyond async #PF. > > > Cc: stable@xxxxxxxxxxxxxxx > > Suggested-by: Ben Gardon <bgardon@xxxxxxxxxx> > > [ Based on a patch from Ben implemented for Google's kernel. ] > > Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx> > > --- > > virt/kvm/kvm_main.c | 8 ++++++++ > > 1 file changed, 8 insertions(+) > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > index 35ae6d32dae5..b59f0a29dbd5 100644 > > --- a/virt/kvm/kvm_main.c > > +++ b/virt/kvm/kvm_main.c > > @@ -117,6 +117,8 @@ EXPORT_SYMBOL_GPL(kvm_debugfs_dir); > > > > static const struct file_operations stat_fops_per_vm; > > > > +static struct file_operations kvm_chardev_ops; > > + > > static long kvm_vcpu_ioctl(struct file *file, unsigned int ioctl, > > unsigned long arg); > > #ifdef CONFIG_KVM_COMPAT > > @@ -1131,6 +1133,11 @@ static struct kvm *kvm_create_vm(unsigned long type) > > preempt_notifier_inc(); > > kvm_init_pm_notifier(kvm); > > > > + if (!try_module_get(kvm_chardev_ops.owner)) { > > The "try" aspect is unnecessary. Stealing from Paolo's version, > > /* KVM is pinned via open("/dev/kvm"), the fd passed to this ioctl(). */ > __module_get(kvm_chardev_ops.owner); Right, I did see that and agree we're guaranteed the KVM module has a reference at this point. But the KVM module might be in state MODULE_STATE_GOING (e.g. if someone ran "rmmod --wait"), which try_module_get() checks. > > > + r = -ENODEV; > > + goto out_err; > > + } > > + > > return kvm; > > > > out_err: > > @@ -1220,6 +1227,7 @@ static void kvm_destroy_vm(struct kvm *kvm) > > preempt_notifier_dec(); > > hardware_disable_all(); > > mmdrop(mm); > > + module_put(kvm_chardev_ops.owner); > > } > > > > void kvm_get_kvm(struct kvm *kvm) > > > > base-commit: b13a3befc815eae574d87e6249f973dfbb6ad6cd > > prerequisite-patch-id: 38f66d60319bf0bc9bf49f91f0f9119e5441629b > > prerequisite-patch-id: 51aa921d68ea649d436ea68e1b8f4aabc3805156 > > -- > > 2.35.1.616.g0bdcbb4464-goog > >