Messages in this thread | | | Subject | Re: [PATCH v2 1/4] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm | From | Leonardo Bras <> | Date | Thu, 14 Nov 2019 15:43:57 -0300 |
| |
On Tue, 2019-11-12 at 15:57 +1100, Michael Ellerman wrote: > Hi Leonardo,
Hello Micheal, thanks for the feedback!
> > 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. >
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 [unhandled content-type:application/pgp-signature] | |