On Tue, 2019-11-12 at 15:57 +1100, Michael Ellerman wrote:

Hi Leonardo,

Hello Micheal, thanks for the feedback!

Leonardo Bras [off-list ref] writes:

quoted

Fixes a possible 'use after free' of kvm variable in
kvm_vm_ioctl_create_spapr_tce, where it does a mutex_unlock(&kvm-

quoted

lock)

after a kvm_put_kvm(kvm).

There is no potential for an actual use after free here AFAICS.

quoted

diff --git a/arch/powerpc/kvm/book3s_64_vio.c

b/arch/powerpc/kvm/book3s_64_vio.c
index 5834db0a54c6..a402ead833b6 100644

--- a/arch/powerpc/kvm/book3s_64_vio.c
+++ b/arch/powerpc/kvm/book3s_64_vio.c

The preceeding context is:

	mutex_lock(&kvm->lock);

	/* Check this LIOBN hasn't been previously allocated */
	ret = 0;
	list_for_each_entry(siter, &kvm->arch.spapr_tce_tables, list) {
		if (siter->liobn == args->liobn) {
			ret = -EBUSY;
			break;
		}
	}

	kvm_get_kvm(kvm);
	if (!ret)
		ret = anon_inode_getfd("kvm-spapr-tce",
&kvm_spapr_tce_fops,
				       stt, O_RDWR | O_CLOEXEC);

quoted

@@ -316,14 +316,13 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm

*kvm,
 
 	if (ret >= 0)
 		list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables);
-	else
-		kvm_put_kvm(kvm);
 
 	mutex_unlock(&kvm->lock);
 
 	if (ret >= 0)
 		return ret;
 
+	kvm_put_kvm(kvm);
 	kfree(stt);
  fail_acct:
 	account_locked_vm(current->mm, kvmppc_stt_pages(npages),
false);

If the kvm_put_kvm() you've moved actually caused the last reference
to
be dropped that would mean that our caller had passed us a kvm struct
without holding a reference to it, and that would be a bug in our
caller.

So, there is no chance that between this function's kvm_get_kvm() and 
kvm_put_kvm(), another thread can decrease this reference counter?

Or put another way, it would mean the mutex_lock() above could
already
be operating on a freed kvm struct.

The kvm_get_kvm() prior to the anon_inode_getfd() is to account for
the
reference that's held by the `stt` struct, and dropped in
kvm_spapr_tce_release().

So although this patch isn't wrong, the explanation is not accurate.

cheers

Kind regards