On 1/12/23 3:38 PM, Matthew Rosato wrote:
> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation. This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
>
> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> group->group_lock/group_rwsem
>
> Avoid this scenario by having vfio core code acquire a KVM reference
> the first time a device is opened and hold that reference until the
> device fd is closed, at a point after the group lock has been released.
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> ---
> Changes from v1:
> * Re-write using symbol get logic to get kvm ref during first device
> open, release the ref during device fd close after group lock is
> released
> * Drop kvm get/put changes to drivers; now that vfio core holds a
> kvm ref until sometime after the device_close op is called, it
> should be fine for drivers to get and put their own references to it.
> ---
> drivers/vfio/group.c | 6 ++---
> drivers/vfio/vfio_main.c | 48 +++++++++++++++++++++++++++++++++++++---
> include/linux/vfio.h | 1 -
> 3 files changed, 48 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
> index bb24b2f0271e..2b0da82f82f4 100644
> --- a/drivers/vfio/group.c
> +++ b/drivers/vfio/group.c
> @@ -165,9 +165,9 @@ static int vfio_device_group_open(struct vfio_device *device)
> }
>
> /*
> - * 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.
> + * Here we pass the KVM pointer with the group under the lock. A
> + * reference will be obtained the first time the device is opened and
> + * will be held until the device fd is closed.
> */
> ret = vfio_device_open(device, device->group->iommufd,
> device->group->kvm);
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index 5177bb061b17..c969e2a0ecd3 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -16,6 +16,7 @@
> #include <linux/fs.h>
> #include <linux/idr.h>
> #include <linux/iommu.h>
> +#include <linux/kvm_host.h>
Ugh, looks like including linux/kvm_host.h here breaks architectures that don't have an arch/*/include/uapi/asm/kvm.h
AFAICT this should be implicit with the CONFIG_HAVE_KVM bool, so unless someone has a better idea, to avoid I think we can key off of CONFIG_HAVE_KVM like so...
#ifdef CONFIG_HAVE_KVM
#include <linux/kvm_host.h>
#endif
[...]
#ifdef CONFIG_HAVE_KVM
[...symbol_get implementation here...]
#else
static bool vfio_kvm_get_kvm_safe(struct kvm *kvm)
{
return false;
}
static void vfio_kvm_put_kvm(struct kvm *kvm)
{
}
#endif
