linux-kernel - [PATCH 1/3] KVM: Properly unwind VM creation if creating debugfs fails

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Date:   Tue, 16 Aug 2022 05:39:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+744e173caec2e1627ee0@...kaller.appspotmail.com,
        Oliver Upton <oliver.upton@...ux.dev>,
        Sean Christopherson <seanjc@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: [PATCH 1/3] KVM: Properly unwind VM creation if creating debugfs fails

Properly unwind VM creation if kvm_create_vm_debugfs() fails.  A recent
change to invoke kvm_create_vm_debug() in kvm_create_vm() was led astray
by buggy try_get_module() handling adding by commit 5f6de5cbebee ("KVM:
Prevent module exit until all VMs are freed").  The debugfs error path
effectively inherits the bad error path of try_module_get(), e.g. KVM
leaves the to-be-free VM on vm_list even though KVM appears to do the
right thing by calling module_put() and falling through.

Opportunistically hoist kvm_create_vm_debugfs() above the call to
kvm_arch_post_init_vm() so that the "post-init" arch hook is actually
invoked after the VM is initialized (ignoring kvm_coalesced_mmio_init()
for the moment).  x86 is the only non-nop implementation of the post-init
hook, and it doesn't allocate/initialize any objects that are reachable
via debugfs code (spawns a kthread worker for the NX huge page mitigation).

Leave the buggy try_get_module() alone for now, it will be fixed in a
separate commit.

Fixes: b74ed7a68ec1 ("KVM: Actually create debugfs in kvm_create_vm()")
Reported-by: syzbot+744e173caec2e1627ee0@...kaller.appspotmail.com
Cc: Oliver Upton <oliver.upton@...ux.dev>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/kvm_main.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 515dfe9d3bcf..ee5f48cc100b 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1211,9 +1211,13 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 	if (r)
 		goto out_err_no_mmu_notifier;

+	r = kvm_create_vm_debugfs(kvm, fdname);
+	if (r)
+		goto out_err_no_debugfs;
+
 	r = kvm_arch_post_init_vm(kvm);
 	if (r)
-		goto out_err_mmu_notifier;
+		goto out_err;

 	mutex_lock(&kvm_lock);
 	list_add(&kvm->vm_list, &vm_list);
@@ -1229,18 +1233,14 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
 	 */
 	if (!try_module_get(kvm_chardev_ops.owner)) {
 		r = -ENODEV;
-		goto out_err_mmu_notifier;
-	}
-
-	r = kvm_create_vm_debugfs(kvm, fdname);
-	if (r)
 		goto out_err;
+	}

 	return kvm;

 out_err:
-	module_put(kvm_chardev_ops.owner);
-out_err_mmu_notifier:
+	kvm_destroy_vm_debugfs(kvm);
+out_err_no_debugfs:
 #if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
 	if (kvm->mmu_notifier.ops)
 		mmu_notifier_unregister(&kvm->mmu_notifier, current->mm);
-- 
2.37.1.595.g718a3a8f04-goog