linux-kernel - Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock

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Message-ID: <Y8CX8YwT/T9v4U/D@google.com>
Date:   Thu, 12 Jan 2023 23:29:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>, pbonzini@...hat.com,
        jgg@...dia.com, cohuck@...hat.com, farman@...ux.ibm.com,
        pmorel@...ux.ibm.com, borntraeger@...ux.ibm.com,
        frankja@...ux.ibm.com, imbrenda@...ux.ibm.com, david@...hat.com,
        akrowiak@...ux.ibm.com, jjherne@...ux.ibm.com, pasic@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vfio: fix potential deadlock on vfio group lock

On Thu, Jan 12, 2023, Matthew Rosato wrote:
> On 1/12/23 4:05 PM, Alex Williamson wrote:
> > On Thu, 12 Jan 2023 15:38:44 -0500
> > Matthew Rosato <mjrosato@...ux.ibm.com> wrote:
> >> @@ -344,6 +345,35 @@ static bool vfio_assert_device_open(struct vfio_device *device)
> >>  	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
> >>  }
> >>  
> >> +static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
> >> +{
> >> +	bool (*fn)(struct kvm *kvm);
> >> +	bool ret;
> >> +
> >> +	fn = symbol_get(kvm_get_kvm_safe);
> >> +	if (WARN_ON(!fn))

In a related vein to Alex's comments about error handling, this should not WARN.
WARNing during vfio_kvm_put_kvm() makes sense, but the "get" is somewhat blind.

> >> +		return false;
> >> +
> >> +	ret = fn(kvm);
> >> +
> >> +	symbol_put(kvm_get_kvm_safe);
> >> +
> >> +	return ret;
> >> +}
> >> +
> >> +static void vfio_kvm_put_kvm(struct kvm *kvm)
> >> +{
> >> +	void (*fn)(struct kvm *kvm);
> >> +
> >> +	fn = symbol_get(kvm_put_kvm);
> >> +	if (WARN_ON(!fn))
> >> +		return;
> >> +
> >> +	fn(kvm);
> >> +
> >> +	symbol_put(kvm_put_kvm);
> >> +}