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>
#include <linux/list.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
@@ -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))
+ 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);
+}
+
static int vfio_device_first_open(struct vfio_device *device,
struct iommufd_ctx *iommufd, struct kvm *kvm)
{
@@ -361,16 +391,24 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;
+ if (kvm && !vfio_kvm_get_kvm_safe(kvm)) {
+ ret = -ENOENT;
+ goto err_unuse_iommu;
+ }
device->kvm = kvm;
if (device->ops->open_device) {
ret = device->ops->open_device(device);
if (ret)
- goto err_unuse_iommu;
+ goto err_put_kvm;
}
return 0;
+err_put_kvm:
+ if (kvm) {
+ vfio_kvm_put_kvm(kvm);
+ device->kvm = NULL;
+ }
err_unuse_iommu:
- device->kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -387,7 +425,6 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
device->ops->close_device(device);
- device->kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -465,6 +502,11 @@ static int vfio_device_fops_release(struct inode *inode, struct file *filep)
vfio_device_group_close(device);
+ if (device->open_count == 0 && device->kvm) {
+ vfio_kvm_put_kvm(device->kvm);
+ device->kvm = NULL;
+ }
+
vfio_device_put_registration(device);
return 0;
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..3ff7e9302cc1 100644
--- a/include/linux/vfio.h
+++ b/include/linux/vfio.h
@@ -46,7 +46,6 @@ struct vfio_device {
struct vfio_device_set *dev_set;
struct list_head dev_set_list;
unsigned int migration_flags;
- /* Driver must reference the kvm during open_device or never touch it */
struct kvm *kvm;
/* Members below here are private, not for driver use */
--
2.39.0
