[Intel-gfx] [PATCH v2] vfio: fix deadlock between group lock and kvm lock

Matthew Rosato
2023-02-01T19:20:28Z

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@linux.ibm.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
---
Changes from v1:
* use spin_lock instead of spin_lock_irqsave (Jason)
* clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
* Re-arrange code to avoid referencing the group contents from within
vfio_main (Kevin) which meant moving most of the code in this patch
to group.c along with getting/dropping of the dev_set lock
---
drivers/vfio/group.c | 90 +++++++++++++++++++++++++++++++++++++---
drivers/vfio/vfio.h | 1 +
drivers/vfio/vfio_main.c | 11 ++---
include/linux/vfio.h | 2 +-
4 files changed, 91 insertions(+), 13 deletions(-)

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f0271e..52f434861294 100644
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -13,6 +13,9 @@
#include <linux/vfio.h>
#include <linux/iommufd.h>
#include <linux/anon_inodes.h>
+#ifdef CONFIG_HAVE_KVM
+#include <linux/kvm_host.h>
+#endif
#include "vfio.h"

static struct vfio {
@@ -154,6 +157,55 @@ static int vfio_group_ioctl_set_container(struct vfio_group *group,
return ret;
}

+#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);
+ 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)
+{
+}
+#endif
+
static int vfio_device_group_open(struct vfio_device *device)
{
int ret;
@@ -164,14 +216,32 @@ static int vfio_device_group_open(struct vfio_device *device)
goto out_unlock;
}

+ mutex_lock(&device->dev_set->lock);
+
/*
- * 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.
+ * Before the first device open, 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 again. Save
+ * the pointer in the device for use by drivers.
*/
+ if (device->open_count == 0) {
+ spin_lock(&device->group->kvm_ref_lock);
+ if (device->group->kvm &&
+ vfio_kvm_get_kvm_safe(device, device->group->kvm))
+ device->kvm = device->group->kvm;
+ spin_unlock(&device->group->kvm_ref_lock);
+ }
+
ret = vfio_device_open(device, device->group->iommufd,
device->group->kvm);

+ if (ret && device->kvm && device->open_count == 0) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ }
+
+ mutex_unlock(&device->dev_set->lock);
+
out_unlock:
mutex_unlock(&device->group->group_lock);
return ret;
@@ -180,7 +250,16 @@ static int vfio_device_group_open(struct vfio_device *device)
void vfio_device_group_close(struct vfio_device *device)
{
mutex_lock(&device->group->group_lock);
+ mutex_lock(&device->dev_set->lock);
+
vfio_device_close(device, device->group->iommufd);
+
+ if (device->kvm && device->open_count == 0) {
+ vfio_kvm_put_kvm(device);
+ device->kvm = NULL;
+ }
+
+ mutex_unlock(&device->dev_set->lock);
mutex_unlock(&device->group->group_lock);
}

@@ -450,6 +529,7 @@ 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;
@@ -803,9 +883,9 @@ void vfio_file_set_kvm(struct file *file, struct kvm *kvm)
if (!vfio_file_is_group(file))
return;

- mutex_lock(&group->group_lock);
+ spin_lock(&group->kvm_ref_lock);
group->kvm = kvm;
- mutex_unlock(&group->group_lock);
+ spin_unlock(&group->kvm_ref_lock);
}
EXPORT_SYMBOL_GPL(vfio_file_set_kvm);

diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
index f8219a438bfb..20c6bc249cb8 100644
--- a/drivers/vfio/vfio.h
+++ b/drivers/vfio/vfio.h
@@ -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..14dbf781ea8c 100644
--- a/drivers/vfio/vfio_main.c
+++ b/drivers/vfio/vfio_main.c
@@ -361,7 +361,6 @@ static int vfio_device_first_open(struct vfio_device *device,
if (ret)
goto err_module_put;

- device->kvm = kvm;
if (device->ops->open_device) {
ret = device->ops->open_device(device);
if (ret)
@@ -370,7 +369,6 @@ static int vfio_device_first_open(struct vfio_device *device,
return 0;

err_unuse_iommu:
- device->kvm = NULL;
if (iommufd)
vfio_iommufd_unbind(device);
else
@@ -387,7 +385,6 @@ 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
@@ -400,14 +397,14 @@ int vfio_device_open(struct vfio_device *device,
{
int ret = 0;

- mutex_lock(&device->dev_set->lock);
+ lockdep_assert_held(&device->dev_set->lock);
+
device->open_count++;
if (device->open_count == 1) {
ret = vfio_device_first_open(device, iommufd, kvm);
if (ret)
device->open_count--;
}
- mutex_unlock(&device->dev_set->lock);

return ret;
}
@@ -415,12 +412,12 @@ int vfio_device_open(struct vfio_device *device,
void vfio_device_close(struct vfio_device *device,
struct iommufd_ctx *iommufd)
{
- mutex_lock(&device->dev_set->lock);
+ lockdep_assert_held(&device->dev_set->lock);
+
vfio_assert_device_open(device);
if (device->open_count == 1)
vfio_device_last_close(device, iommufd);
device->open_count--;
- mutex_unlock(&device->dev_set->lock);
}

/*
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