[PATCH v4] vfio: fix potential deadlock on vfio group lock

Matthew Rosato
2023-01-14T00:04:06Z

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 right
after the group lock is released after the last device is closed.

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 v3:
* Can't check for open_count after the group lock has been dropped because
it would be possible for the count to change again once the group lock
is dropped (Jason)
Solve this by stashing a copy of the kvm and put_kvm while the group
lock is held, nullifying the device copies of these in device_close()
as soon as the open_count reaches 0, and then checking to see if the
device->kvm changed before dropping the group lock. If it changed
during close, we can drop the reference using the stashed kvm and put
function after dropping the group lock.

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 | 23 +++++++++++++--
drivers/vfio/vfio.h | 9 ++++++
drivers/vfio/vfio_main.c | 61 +++++++++++++++++++++++++++++++++++++---
include/linux/vfio.h | 2 +-
4 files changed, 87 insertions(+), 8 deletions(-)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..b396c17d7390 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 open_count reaches 0.
*/
ret = vfio_device_open(device, device->group->iommufd,
device->group->kvm);
@@ -179,9 +179,26 @@ static int vfio_device_group_open(struct vfio_device *device)

void vfio_device_group_close(struct vfio_device *device)
{
+ void (*put_kvm)(struct kvm *kvm);
+ struct kvm *kvm;
+
mutex_lock(&device->group->group_lock);
+ kvm = device->kvm;
+ put_kvm = device->put_kvm;
vfio_device_close(device, device->group->iommufd);
+ if (kvm == device->kvm)
+ kvm = NULL;
mutex_unlock(&device->group->group_lock);
+
+ /*
+ * The last kvm reference will trigger kvm_destroy_vm, which can in
+ * turn re-enter vfio and attempt to acquire the group lock. Therefore
+ * we get a copy of the kvm pointer and the put function under the
+ * group lock but wait to put that reference until after releasing the
+ * lock.
+ */
+ if (kvm)
+ vfio_kvm_put_kvm(put_kvm, kvm);
}

static struct file *vfio_device_open_file(struct vfio_device *device)
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..08a5a23d6fef 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -251,4 +251,13 @@ extern bool vfio_noiommu __read_mostly;
enum { vfio_noiommu = false };
#endif

+#ifdef CONFIG_HAVE_KVM
+void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm);
+#else
+static inline void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm),
+ struct kvm *kvm)
+{
+}
+#endif
+
#endif
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..c6bb07af46b8 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 +347,49 @@ 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;
+}
+
+void vfio_kvm_put_kvm(void (*put)(struct kvm *kvm), struct kvm *kvm)
+{
+ if (WARN_ON(!put))
+ return;
+
+ put(kvm);
+ symbol_put(kvm_put_kvm);
+}
+#else
+static bool vfio_kvm_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+ return false;
+}
+#endif
+
static int vfio_device_first_open(struct vfio_device *device,
struct iommufd_ctx *iommufd, struct kvm *kvm)
{
@@ -361,16 +407,22 @@ 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->put_kvm, device->kvm);
+ device->put_kvm = NULL;
+ device->kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -388,6 +440,7 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
device->ops->close_device(device);
device->kvm = NULL;
+ device->put_kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
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