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

Liu, Yi L
2023-02-02T03:47:07Z

> From: Alex Williamson <alex.williamson@redhat.com>
> Sent: Thursday, February 2, 2023 7:28 AM
>
> On Wed, 1 Feb 2023 14:20:10 -0500
> Matthew Rosato <mjrosato@linux.ibm.com> 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@linux.ibm.com>
> > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> > ---
> > 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