[PATCH v3] vfio: fix potential deadlock on vfio group lock

[Matthew Rosato <mjrosato@linux.ibm.com>]

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@redhat.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
Changes from v2:
* Re-arrange vfio_kvm_set_kvm_safe error path to still trigger
  device_open with device->kvm=NULL (Alex)
* get device->dev_set->lock when checking device->open_count (Alex)
* but don't hold it over the kvm_put_kvm (Jason)
* get kvm_put symbol upfront and stash it in device until close (Jason)
* check CONFIG_HAVE_KVM to avoid build errors on architectures without
  KVM support

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 | 78 +++++++++++++++++++++++++++++++++++++---
 include/linux/vfio.h     |  2 +-
 3 files changed, 77 insertions(+), 9 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..dbdf16903d52 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -16,6 +16,9 @@
 #include <linux/fs.h>
 #include <linux/idr.h>
 #include <linux/iommu.h>
+#ifdef CONFIG_HAVE_KVM
+#include <linux/kvm_host.h>
+#endif
 #include <linux/list.h>
 #include <linux/miscdevice.h>
 #include <linux/module.h>
@@ -344,6 +347,57 @@ static bool vfio_assert_device_open(struct vfio_device *device)
 	return !WARN_ON_ONCE(!READ_ONCE(device->open_count));
 }
 
+#ifdef CONFIG_HAVE_KVM
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+	void (*pfn)(struct kvm *kvm);
+	bool (*fn)(struct kvm *kvm);
+	bool ret;
+
+	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, struct kvm *kvm)
+{
+	if (WARN_ON(!device->put_kvm))
+		return;
+
+	device->put_kvm(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, struct kvm *kvm)
+{
+
+}
+#endif
+
 static int vfio_device_first_open(struct vfio_device *device,
 				  struct iommufd_ctx *iommufd, struct kvm *kvm)
 {
@@ -361,16 +415,21 @@ static int vfio_device_first_open(struct vfio_device *device,
 	if (ret)
 		goto err_module_put;
 
-	device->kvm = kvm;
+	if (kvm && vfio_kvm_get_kvm_safe(device, kvm))
+		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_unuse_iommu:
-	device->kvm = NULL;
+err_put_kvm:
+	if (device->kvm) {
+		vfio_kvm_put_kvm(device, device->kvm);
+		device->kvm = NULL;
+	}
 	if (iommufd)
 		vfio_iommufd_unbind(device);
 	else
@@ -387,7 +446,6 @@ 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
@@ -462,9 +520,19 @@ static inline void vfio_device_pm_runtime_put(struct vfio_device *device)
 static int vfio_device_fops_release(struct inode *inode, struct file *filep)
 {
 	struct vfio_device *device = filep->private_data;
+	struct kvm *kvm = NULL;
 
 	vfio_device_group_close(device);
 
+	mutex_lock(&device->dev_set->lock);
+	if (device->open_count == 0 && device->kvm) {
+		kvm = device->kvm;
+		device->kvm = NULL;
+	}
+	mutex_unlock(&device->dev_set->lock);
+	if (kvm)
+		vfio_kvm_put_kvm(device, kvm);
+
 	vfio_device_put_registration(device);
 
 	return 0;
diff --git a/include/linux/vfio.h b/include/linux/vfio.h
index 35be78e9ae57..87ff862ff555 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 */
@@ -58,6 +57,7 @@ struct vfio_device {
 	struct list_head group_next;
 	struct list_head iommu_entry;
 	struct iommufd_access *iommufd_access;
+	void (*put_kvm)(struct kvm *kvm);
 #if IS_ENABLED(CONFIG_IOMMUFD)
 	struct iommufd_device *iommufd_device;
 	struct iommufd_ctx *iommufd_ictx;
-- 
2.39.0