RE: [PATCH v2] vfio: fix deadlock between group lock and kvm lock

[Liu, Yi L]

> From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Sent: Thursday, February 2, 2023 7:28 AM
> 
> On Wed,  1 Feb 2023 14:20:10 -0500
> Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
> 
> > After 51cdc8bc120e, we have another deadlock scenario between the
> > kvm->lock and the vfio group_lock with two different codepaths acquiring
> > the locks in different order.  Specifically in vfio_open_device, vfio
> > holds the vfio group_lock when issuing device->ops->open_device but
> some
> > drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> > routine;  Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> > before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> >
> > To resolve this, let's remove the need for the vfio group_lock from the
> > kvm_vfio_release codepath.  This is done by introducing a new spinlock to
> > protect modifications to the vfio group kvm pointer, and acquiring a kvm
> > ref from within vfio while holding this spinlock, with the reference held
> > until the last close for the device in question.
> >
> > Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> > Reported-by: Anthony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> > ---
> > Changes from v1:
> > * use spin_lock instead of spin_lock_irqsave (Jason)
> > * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
> > * Re-arrange code to avoid referencing the group contents from within
> >   vfio_main (Kevin) which meant moving most of the code in this patch
> >   to group.c along with getting/dropping of the dev_set lock
> > ---
> >  drivers/vfio/group.c     | 90
> +++++++++++++++++++++++++++++++++++++---
> >  drivers/vfio/vfio.h      |  1 +
> >  drivers/vfio/vfio_main.c | 11 ++---
> >  include/linux/vfio.h     |  2 +-
> >  4 files changed, 91 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> > index bb24b2f0271e..52f434861294 100644
> > --- a/drivers/vfio/group.c
> > +++ b/drivers/vfio/group.c
> > @@ -13,6 +13,9 @@
> >  #include <linux/vfio.h>
> >  #include <linux/iommufd.h>
> >  #include <linux/anon_inodes.h>
> > +#ifdef CONFIG_HAVE_KVM
> > +#include <linux/kvm_host.h>
> > +#endif
> >  #include "vfio.h"
> >
> >  static struct vfio {
> > @@ -154,6 +157,55 @@ static int vfio_group_ioctl_set_container(struct
> vfio_group *group,
> >  	return ret;
> >  }
> >
> > +#ifdef CONFIG_HAVE_KVM
> > +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct
> kvm *kvm)
> 
> I'm tempted to name these vfio_device_get_kvm_safe() and only pass the
> vfio_device, where of course we can get the kvm pointer from the group
> internally.
> 
> > +{
> > +	void (*pfn)(struct kvm *kvm);
> > +	bool (*fn)(struct kvm *kvm);
> > +	bool ret;
> > +
> 
> We should assert_lockdep_held(&device->dev_set->lock) in both of these
> since that seems to be what's protecting device->kvm and
> device->put_kvm.
> 
> If we change as above to get the kvm pointer from the group within this
> function, we can also move the kvm_ref_lock here, which seems to
> simplify the caller quite a bit.
> 
> > +	pfn = symbol_get(kvm_put_kvm);
> > +	if (WARN_ON(!pfn))
> > +		return false;
> > +
> > +	fn = symbol_get(kvm_get_kvm_safe);
> > +	if (WARN_ON(!fn)) {
> > +		symbol_put(kvm_put_kvm);
> > +		return false;
> > +	}
> > +
> > +	ret = fn(kvm);
> > +	if (ret)
> > +		device->put_kvm = pfn;
> > +	else
> > +		symbol_put(kvm_put_kvm);
> > +
> > +	symbol_put(kvm_get_kvm_safe);
> > +
> > +	return ret;
> > +}
> > +
> > +static void vfio_kvm_put_kvm(struct vfio_device *device)
> > +{
> > +	if (WARN_ON(!device->kvm || !device->put_kvm))
> > +		return;
> 
> It simplifies the caller if we can use this even in the !device->kvm
> case.
> 
> > +
> > +	device->put_kvm(device->kvm);
> > +	device->put_kvm = NULL;
> > +	symbol_put(kvm_put_kvm);
> > +}
> > +
> > +#else
> > +static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct
> kvm *kvm)
> > +{
> > +	return false;
> > +}
> > +
> > +static void vfio_kvm_put_kvm(struct vfio_device *device)
> > +{
> > +}
> > +#endif
> > +
> >  static int vfio_device_group_open(struct vfio_device *device)
> >  {
> >  	int ret;
> > @@ -164,14 +216,32 @@ static int vfio_device_group_open(struct
> vfio_device *device)
> >  		goto out_unlock;
> >  	}
> >
> > +	mutex_lock(&device->dev_set->lock);
> > +
> >  	/*
> > -	 * Here we pass the KVM pointer with the group under the lock.  If
> the
> > -	 * device driver will use it, it must obtain a reference and release it
> > -	 * during close_device.
> > +	 * Before the first device open, get the KVM pointer currently
> > +	 * associated with the group (if there is one) and obtain a reference
> > +	 * now that will be held until the open_count reaches 0 again.  Save
> > +	 * the pointer in the device for use by drivers.
> >  	 */
> > +	if (device->open_count == 0) {
> > +		spin_lock(&device->group->kvm_ref_lock);
> > +		if (device->group->kvm &&
> > +		    vfio_kvm_get_kvm_safe(device, device->group->kvm))
> > +			device->kvm = device->group->kvm;
> > +		spin_unlock(&device->group->kvm_ref_lock);
> > +	}
> > +
> >  	ret = vfio_device_open(device, device->group->iommufd,
> >  			       device->group->kvm);
> 
> We're using device->group->kvm outside of kvm_ref_lock here, it should
> be using device->kvm.

Existing code set device->kvm in the vfio_device_first_open() which is
called by vfio_device_open(). After above change, seems not necessary
to pass kvm pointer into the call chain. Isn't it?

Regards,
Yi Liu