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@syzkaller.appspotmail.com
Cc: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Message-Id: <20220816053937.2477106-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 515dfe9d3bcfb09b091f6af83dae7d76210f60dd..ee5f48cc100b1512223de4d6bf9bb1e461ff64d0 100644 (file)
--- 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);