linux-kernel - [PATCH] KVM: allow struct kvm to outlive the file descriptors

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Message-Id: <20220302174321.326189-1-pbonzini@redhat.com>
Date:   Wed,  2 Mar 2022 12:43:21 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     dmatlack@...gle.com, seanjc@...gle.com
Subject: [PATCH] KVM: allow struct kvm to outlive the file descriptors

Right now, the kvm module is kept alive by VFS via fops_get/fops_put, but there
may be cases in which a kvm_get_kvm's matching kvm_put_kvm happens after
the file descriptor is closed.  One case that will be introduced soon is
when work is delegated to the system work queue; the worker might be
a bit late and the file descriptor can be closed in the meantime.  Ensure
that the module has not gone away by tying a module reference explicitly
to the lifetime of the struct kvm.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 virt/kvm/kvm_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 64eb99444688..e3f37fc2ebf1 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1131,6 +1131,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
 	preempt_notifier_inc();
 	kvm_init_pm_notifier(kvm);

+	/* This is safe, since we have a reference from open(). */
+	__module_get(THIS_MODULE);
+
 	return kvm;

 out_err:
@@ -1220,6 +1223,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	preempt_notifier_dec();
 	hardware_disable_all();
 	mmdrop(mm);
+	module_put(THIS_MODULE);
 }

 void kvm_get_kvm(struct kvm *kvm)
-- 
2.31.1