On Thu, 2 Feb 2023 03:46:59 +0000 "Liu, Yi L" <yi.l.liu@xxxxxxxxx> wrote: > > 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? Yes, we can get it from the device. I didn't check how much this bloats the patch though. As a fix, it might make sense to save that refactoring for a follow-on patch. Thanks, Alex