[PATCH kernel] KVM: PPC: Improve KVM reference counting

[Alexey Kardashevskiy <aik@ozlabs.ru>]

The anon fd's ops releases the KVM reference in the release hook.
However we reference the KVM object after we create the fd so there is
small window when the release function can be called and
dereferenced the KVM object which potentially may free it.

It is not a problem at the moment as the file is created and KVM is
referenced under the KVM lock and the release function obtains the same
lock before dereferencing the KVM (although the lock is not held when
calling kvm_put_kvm()) but it is a fragile against future changes.

This references the KVM object before creating a file.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---

The original bug is described here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cfa393811
But in this case kvm_put_kvm() is called straight away with no locks before/after/around.

---
 arch/powerpc/kvm/book3s_64_vio.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
index 532ab797..d68c969 100644
--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c
@@ -338,14 +338,15 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
 		}
 	}
 
+	kvm_get_kvm(kvm);
 	if (!ret)
 		ret = anon_inode_getfd("kvm-spapr-tce", &kvm_spapr_tce_fops,
 				       stt, O_RDWR | O_CLOEXEC);
 
-	if (ret >= 0) {
+	if (ret >= 0)
 		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
-		kvm_get_kvm(kvm);
-	}
+	else
+		kvm_put_kvm(kvm);
 
 	mutex_unlock(&kvm->lock);
 
-- 
2.17.1