linux-kernel - Re: [PATCH v2 1/2] KVM: destruct kvm_io_device while unregistering it from kvm_io

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Message-ID: <ZBxzphnyLPwBimKL@google.com>
Date:   Thu, 23 Mar 2023 08:43:34 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     pbonzini@...hat.com, mhal@...x.co, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] KVM: destruct kvm_io_device while unregistering it
 from kvm_io_bus

On Tue, Feb 07, 2023, Wei Wang wrote:
> Current usage of kvm_io_device requires users to destruct it with an extra
> call of kvm_iodevice_destructor after the device gets unregistered from
> kvm_io_bus. This is not necessary and can cause errors if a user forgot
> to make the extra call.
> 
> Simplify the usage by combining kvm_iodevice_destructor into
> kvm_io_bus_unregister_dev. This reduces LOCs a bit for users and can
> avoid the leakage of destructing the device explicitly.

The changelog should really call out that coalesced_mmio_ops and ioeventfd_ops
are the only kvm_io_device_ops instances that implement ->destructor.  Without
that info, this change looks super dangerous as it's not obvious other paths won't
end up with a use-after-free.

Paolo, if/when you take this, can you tack on something like:

Note, coalesced_mmio_ops and ioeventfd_ops are the only instances of
kvm_io_device_ops that implement a destructor, all other callers of
kvm_io_bus_unregister_dev() are unaffected by this change.

> Signed-off-by: Wei Wang <wei.w.wang@...el.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@...gle.com>