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