From mboxrd@z Thu Jan 1 00:00:00 1970 From: Leonardo Bras Date: Thu, 14 Nov 2019 18:43:57 +0000 Subject: Re: [PATCH v2 1/4] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm Message-Id: MIME-Version: 1 Content-Type: multipart/mixed; boundary="=-zmSym6hLOrd0Lb87qEYc" List-Id: References: <20191107170258.36379-1-leonardo@linux.ibm.com> <20191107170258.36379-2-leonardo@linux.ibm.com> <87mud13d4r.fsf@mpe.ellerman.id.au> In-Reply-To: <87mud13d4r.fsf@mpe.ellerman.id.au> To: Michael Ellerman , kvm-ppc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org Cc: Paul Mackerras , Benjamin Herrenschmidt --=-zmSym6hLOrd0Lb87qEYc Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, 2019-11-12 at 15:57 +1100, Michael Ellerman wrote: > Hi Leonardo, Hello Micheal, thanks for the feedback! >=20 > Leonardo Bras 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). >=20 > There is no potential for an actual use after free here AFAICS. >=20 > > 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 >=20 > The preceeding context is: >=20 > mutex_lock(&kvm->lock); >=20 > /* Check this LIOBN hasn't been previously allocated */ > ret =3D 0; > list_for_each_entry(siter, &kvm->arch.spapr_tce_tables, list) { > if (siter->liobn =3D=3D args->liobn) { > ret =3D -EBUSY; > break; > } > } >=20 > kvm_get_kvm(kvm); > if (!ret) > ret =3D anon_inode_getfd("kvm-spapr-tce", > &kvm_spapr_tce_fops, > stt, O_RDWR | O_CLOEXEC); >=20 > > @@ -316,14 +316,13 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm > > *kvm, > > =20 > > if (ret >=3D 0) > > list_add_rcu(&stt->list, &kvm->arch.spapr_tce_tables); > > - else > > - kvm_put_kvm(kvm); > > =20 > > mutex_unlock(&kvm->lock); > > =20 > > if (ret >=3D 0) > > return ret; > > =20 > > + kvm_put_kvm(kvm); > > kfree(stt); > > fail_acct: > > account_locked_vm(current->mm, kvmppc_stt_pages(npages), > > false); >=20 > 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. >=20 So, there is no chance that between this function's kvm_get_kvm() and=20 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. >=20 > 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(). >=20 > So although this patch isn't wrong, the explanation is not accurate. >=20 > cheers Kind regards --=-zmSym6hLOrd0Lb87qEYc Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEMdeUgIzgjf6YmUyOlQYWtz9SttQFAl3NoG0ACgkQlQYWtz9S ttT2MBAAw0s38wOkqRjRWeoZ0TV3qd7Gkqzy4gGKHiDNL82fHh6O6X4JSLTYP11u co3J9O1jORkIG7AAHQh68iiGJZY6MB5ZUJkUsn3XVH2gMWcQ/547HpcgCCMkrL8k keCqL7CKz5B7WxKK+8daUvMaTPAY5AaSEOHIHG/In4vEeHuUWJEgBN60tGxvhc1m 7VIEBySrLqKeAgs70rThjBVaqg+66SbLZb7ToVIcoRu3Jc/3O4HHG+f6SnD3tqDM E3BrLge33ZTBt32TdKyogJ6RMUC+SXQXbXfUeNwjXuQIkH3D/zekc31o1GjcenCU Sv0z3Bmcgz2av0FxrA36K7Ch8Mgcnt7Yk5oHwF76kzOMSYyZMw9mJgTwrOHUsxnQ pH73v2e9ol1vDD+bdqkGBpusQae25A3/CDLZPXKvRdSqyaaI3yhwNMJ4deh8Glix bMpjCaUO+Q1o+VuhujYvmB7+tDYSEjWhOgo7m4yuffBu/MDtbaMkbkuqm66mrqrf 3582+Kxt8nEWePhyVBKuJTC+IBqqZEIFO7VCR/XPnac7dYhIOUug4hj2QuYuMStF Bgsqa9pPIECX07kZR4LUVaEfV+uKadq5DcbdAhfR6NlFBgrJn+XBiuCSIalD7G3D bC6v7rRE4tjEoO12FYm165cYeD8I30YqLqkR7/z5OmbB9KkSAj8= =alk4 -----END PGP SIGNATURE----- --=-zmSym6hLOrd0Lb87qEYc--