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.
SGTM. It's a moot point in terms of stable inclusion since
af585b921e5d was first added in v2.6.38. But for anyone doing their
own backporting, 3d3aab1b973b makes it a bit more obvious this is a
generic problem even though it's not the commit that introduces the
bug.
>
> > 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);
>
> > + 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
> >
