linux-kernel - Re: [PATCH 1/1] powerpc/kvm/book3s: Fixes possible 'use after release' of kvm

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Message-ID: <5789f26ca5568f05b9554053a1c6c3309d5c36e8.camel@linux.ibm.com>
Date:   Thu, 28 Nov 2019 13:24:22 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     Paul Mackerras <paulus@...abs.org>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/1] powerpc/kvm/book3s: Fixes possible 'use after
 release' of kvm

On Thu, 2019-11-28 at 09:57 +1100, Paul Mackerras wrote:
> There isn't a potential use-after-free here.  We are relying on the
> property that the release function (kvm_vm_release) cannot be called
> in parallel with this function.  The reason is that this function
> (kvm_vm_ioctl_create_spapr_tce) is handling an ioctl on a kvm VM file
> descriptor.  That means that a userspace process has the file
> descriptor still open.  The code that implements the close() system
> call makes sure that no thread is still executing inside any system
> call that is using the same file descriptor before calling the file
> descriptor's release function (in this case, kvm_vm_release).  That
> means that this kvm_put_kvm() call here cannot make the reference
> count go to zero.

That was very informative. A lot of things are clear to me now.
Thanks for explaining this Paul. 

Best regards,
Leonardo

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