Messages in this thread Patch in this message | | | From | Michael Ellerman <> | Subject | Re: [PATCH v2 1/4] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm | Date | Tue, 12 Nov 2019 15:57:40 +1100 |
| |
Hi Leonardo,
Leonardo Bras <leonardo@linux.ibm.com> writes: > Fixes a possible 'use after free' of kvm variable in > kvm_vm_ioctl_create_spapr_tce, where it does a mutex_unlock(&kvm->lock) > after a kvm_put_kvm(kvm).
There is no potential for an actual use after free here AFAICS.
> 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);
> @@ -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.
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
| |