[PATCH RESEND 1/2] KVM: Prevent module exit until all VMs are freed

[David Matlack <dmatlack@google.com>]

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;
+	}
+
 	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