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Message-ID: <c792fdc629d87f452d4348d33ab179df01d42017.camel@linux.ibm.com>
Subject: Re: [PATCH v2 1/4] powerpc/kvm/book3s: Fixes possible 'use after
 release' of kvm
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     Michael Ellerman <mpe@ellerman.id.au>, kvm-ppc@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Cc:     Paul Mackerras <paulus@ozlabs.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>
Date:   Thu, 14 Nov 2019 15:43:57 -0300
In-Reply-To: <87mud13d4r.fsf@mpe.ellerman.id.au>
References: <20191107170258.36379-1-leonardo@linux.ibm.com>
         <20191107170258.36379-2-leonardo@linux.ibm.com>
         <87mud13d4r.fsf@mpe.ellerman.id.au>
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On Tue, 2019-11-12 at 15:57 +1100, Michael Ellerman wrote:
> Hi Leonardo,

Hello Micheal, thanks for the feedback!

>=20
> 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).
>=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

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