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Message-ID: <41fe3962ce1f1d5f61db5f5c28584f68ad66b2b1.camel@linux.ibm.com>
Subject: Re: [PATCH] KVM: Add separate helper for putting borrowed reference
 to kvm
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Radim =?UTF-8?Q?Kr=C4=8Dm=C3=A1=C5=99?= <rkrcmar@redhat.com>,
        kvm-ppc@vger.kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Date:   Wed, 27 Nov 2019 18:57:10 -0300
In-Reply-To: <103b290917221baa10194c27c8e35b9803f3cafa.camel@linux.ibm.com>
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On Wed, 2019-11-27 at 17:15 -0300, Leonardo Bras wrote:
> > > > > So, suppose these threads, where:
> > > > > - T1 uses a borrowed reference, and=20
> > > > > - T2 is releasing the reference (close, release):
> > > >=20
> > > > Nit: T2 is releasing the *last* reference (as implied by your refer=
ence
> > > > to close/release).
> > >=20
> > > Correct.
> > >=20
> > > > > T1                              | T2
> > > > > kvm_get_kvm()                   |
> > > > > ...                             | kvm_put_kvm()
> > > > > kvm_put_kvm_no_destroy()        |
> > > > >=20
> > > > > The above would not trigger a use-after-free bug, but will cause =
a
> > > > > memory leak. Is my above understanding right?
> > > >=20
> > > > Yes, this is correct.
> > > >=20
> > >=20
> > > Then, what would not be a bug before (using kvm_put_kvm()) now is a
> > > memory leak (using kvm_put_kvm_no_destroy()).
> >=20

Sorry, I missed some information on above example.=20
Suppose on that example that the reorder changes take place so that
kvm_put_kvm{,_no_destroy}() always happens after the last usage of kvm
(in the same syscall, let's say).

Before T1 and T2, refcount =3D 1;

If T1 uses kvm_put_kvm_no_destroy():
- T1 increases refcount (=3D2)
- T2 decreases refcount (=3D1)
- T1 decreases refcount, (=3D0) don't free kvm (memleak)

If T1 uses kvm_put_kvm():
- T1 increases refcount (=3D 2)
- T2 decreases refcount (=3D 1)
- T1 decreases refcount, (=3D 0) frees kvm.

So using kvm_put_kvm_no_destroy() would introduce a memleak where it
would have no bug.

> > No, using kvm_put_kvm_no_destroy() changes how a bug would manifest, as
> > you note below.  Replacing kvm_put_kvm() with kvm_put_kvm_no_destroy()
> > when the refcount is _guaranteed_ to be >1 has no impact on correctness=
.

Yes, you are correct.=20
But on the above case, kvm_put_kvm{,_no_destroy}() would be called
with refcount =3D=3D 1, and if reorder patch is applied, it would not cause
any use-after-free error, even on kvm_put_kvm() case.

Is the above correct?

Best regards,

Leonardo


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