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>
---
drivers/vfio/group.c | 14 +++----
drivers/vfio/vfio.h | 5 ++-
drivers/vfio/vfio_main.c | 88 ++++++++++++++++++++++++++++++++++++----
include/linux/vfio.h | 2 +-
4 files changed, 88 insertions(+), 21 deletions(-)
diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..8f67d33e0e8d 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -164,13 +164,7 @@ static int vfio_device_group_open(struct vfio_device *device)
goto out_unlock;
}
- /*
- * 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.
- */
- ret = vfio_device_open(device, device->group->iommufd,
- device->group->kvm);
+ ret = vfio_device_open(device, device->group);
out_unlock:
mutex_unlock(&device->group->group_lock);
@@ -450,6 +444,7 @@ static struct vfio_group *vfio_group_alloc(struct iommu_group *iommu_group,
refcount_set(&group->drivers, 1);
mutex_init(&group->group_lock);
+ spin_lock_init(&group->kvm_ref_lock);
INIT_LIST_HEAD(&group->device_list);
mutex_init(&group->device_lock);
group->iommu_group = iommu_group;
@@ -799,13 +794,14 @@ EXPORT_SYMBOL_GPL(vfio_file_enforced_coherent);
void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
{
struct vfio_group *group = file->private_data;
+ unsigned long flags;
if (!vfio_file_is_group(file))
return;
- mutex_lock(&group->group_lock);
+ spin_lock_irqsave(&group->kvm_ref_lock, flags);
group->kvm = kvm;
- mutex_unlock(&group->group_lock);
+ spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
}
EXPORT_SYMBOL_GPL(vfio_file_set_kvm);
diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..d153ad35de24 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -15,11 +15,11 @@ struct iommufd_ctx;
struct iommu_group;
struct vfio_device;
struct vfio_container;
+struct vfio_group;
void vfio_device_put_registration(struct vfio_device *device);
bool vfio_device_try_get_registration(struct vfio_device *device);
-int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm);
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group);
void vfio_device_close(struct vfio_device *device,
struct iommufd_ctx *iommufd);
@@ -74,6 +74,7 @@ struct vfio_group {
struct file *opened_file;
struct blocking_notifier_head notifier;
struct iommufd_ctx *iommufd;
+ spinlock_t kvm_ref_lock;
};
int vfio_device_set_group(struct vfio_device *device,
diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb061b17..b0defb0d0d87 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>
@@ -344,9 +347,59 @@ 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)
+{
+ if (WARN_ON(!device->kvm || !device->put_kvm))
+ return;
+
+ device->put_kvm(device->kvm);
+ 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_first_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+ struct vfio_group *group)
{
+ struct iommufd_ctx *iommufd = group->iommufd;
+ unsigned long flags;
int ret;
lockdep_assert_held(&device->dev_set->lock);
@@ -361,16 +414,30 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;
- device->kvm = kvm;
+ /*
+ * 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. Save the pointer in the device for use by
+ * drivers.
+ */
+ spin_lock_irqsave(&group->kvm_ref_lock, flags);
+ if (group->kvm && vfio_kvm_get_kvm_safe(device, group->kvm))
+ device->kvm = group->kvm;
+ spin_unlock_irqrestore(&group->kvm_ref_lock, flags);
+
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->put_kvm = NULL;
+ device->kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -387,7 +454,11 @@ static void vfio_device_last_close(struct vfio_device *device,
if (device->ops->close_device)
device->ops->close_device(device);
- device->kvm = NULL;
+ if (device->kvm) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ device->put_kvm = NULL;
+ }
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -395,15 +466,14 @@ static void vfio_device_last_close(struct vfio_device *device,
module_put(device->dev->driver->owner);
}
-int vfio_device_open(struct vfio_device *device,
- struct iommufd_ctx *iommufd, struct kvm *kvm)
+int vfio_device_open(struct vfio_device *device, struct vfio_group *group)
{
int ret = 0;
mutex_lock(&device->dev_set->lock);
device->open_count++;
if (device->open_count == 1) {
- ret = vfio_device_first_open(device, iommufd, kvm);
+ ret = vfio_device_first_open(device, group);
if (ret)
device->open_count--;
}
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;
--
2.39.1
