Hi, David.
Some comments below.
On 3/3/22 15:33, 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") 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: David Matlack dmatlack@google.com
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)) {
r = -ENODEV;goto out_err;- }
Doesn't this problem also affects the other functions called from kvm_dev_ioctl()?
Is it possible that the module is removed while other ioctl's are still running, e.g. KVM_GET_API_VERSION and KVM_CHECK_EXTENSION, even though they don't use struct kvm?
I wonder if this try_module_get() (along with module_put() in the out path of the function) shouldn't be placed in the upper function kvm_dev_ioctl() so it would cover all the other ioctl's.
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