linux-kernel - [PATCH 5.16 0904/1017] KVM: Prevent module exit until all VMs are freed

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Message-Id: <20220405070421.058915912@linuxfoundation.org>
Date:   Tue,  5 Apr 2022 09:30:17 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH 5.16 0904/1017] KVM: Prevent module exit until all VMs are freed

From: David Matlack <dmatlack@...gle.com>

commit 5f6de5cbebee925a612856fce6f9182bb3eee0db upstream.

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")
Fixes: 3d3aab1b973b ("KVM: set owner of cpu and vm file operations")
Cc: stable@...r.kernel.org
Suggested-by: Ben Gardon <bgardon@...gle.com>
[ Based on a patch from Ben implemented for Google's kernel. ]
Signed-off-by: David Matlack <dmatlack@...gle.com>
Message-Id: <20220303183328.1499189-2-dmatlack@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 virt/kvm/kvm_main.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

--- 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
@@ -1107,6 +1109,16 @@ static struct kvm *kvm_create_vm(unsigne
 	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:
@@ -1196,6 +1208,7 @@ static void kvm_destroy_vm(struct kvm *k
 	preempt_notifier_dec();
 	hardware_disable_all();
 	mmdrop(mm);
+	module_put(kvm_chardev_ops.owner);
 }

 void kvm_get_kvm(struct kvm *kvm)