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KVM: Prevent module exit until all VMs are freed

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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: af585b921e ("KVM: Halt vcpu if page it tries to access is swapped out")
Fixes: 3d3aab1b97 ("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: David Matlack <dmatlack@google.com>
Message-Id: <20220303183328.1499189-2-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
David Matlack 2022-03-03 18:33:27 +00:00 committed by Paolo Bonzini
parent c9b8fecddb
commit 5f6de5cbeb
1 changed files with 13 additions and 0 deletions
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13
virt/kvm/kvm_main.c
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@ -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 +1133,16 @@ static struct kvm *kvm_create_vm(unsigned long type)
preempt_notifier_inc();
kvm_init_pm_notifier(kvm);
/*
* When the fd passed to this ioctl() is opened it pins the module,
* but try_module_get() also prevents getting a reference if the module
* is in MODULE_STATE_GOING (e.g. if someone ran "rmmod --wait").
*/
if (!try_module_get(kvm_chardev_ops.owner)) {
r = -ENODEV;
goto out_err;
}
return kvm;
out_err:
@ -1220,6 +1232,7 @@ 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)