vfio: fix deadlock between group lock and kvm lock

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>
Tested-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20230203215027.151988-2-mjrosato@linux.ibm.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

diff --git a/drivers/vfio/group.c b/drivers/vfio/group.c
index bb24b2f..98621ac 100644 (file)
--- a/drivers/vfio/group.c
+++ b/drivers/vfio/group.c
@@ -154,6 +154,18 @@ out_unlock:
        return ret;
 }
 
+static void vfio_device_group_get_kvm_safe(struct vfio_device *device)
+{
+       spin_lock(&device->group->kvm_ref_lock);
+       if (!device->group->kvm)
+               goto unlock;
+
+       _vfio_device_get_kvm_safe(device, device->group->kvm);
+
+unlock:
+       spin_unlock(&device->group->kvm_ref_lock);
+}
+
 static int vfio_device_group_open(struct vfio_device *device)
 {
        int ret;
@@ -164,13 +176,23 @@ 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.
         */
-       ret = vfio_device_open(device, device->group->iommufd,
-                              device->group->kvm);
+       if (device->open_count == 0)
+               vfio_device_group_get_kvm_safe(device);
+
+       ret = vfio_device_open(device, device->group->iommufd, device->kvm);
+
+       if (device->open_count == 0)
+               vfio_device_put_kvm(device);
+
+       mutex_unlock(&device->dev_set->lock);
 
 out_unlock:
        mutex_unlock(&device->group->group_lock);
@@ -180,7 +202,14 @@ out_unlock:
 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->open_count == 0)
+               vfio_device_put_kvm(device);
+
+       mutex_unlock(&device->dev_set->lock);
        mutex_unlock(&device->group->group_lock);
 }
 
@@ -450,6 +479,7 @@ 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 +833,9 @@ 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 d5fa896..8ec3a5d 100644 (file)
--- 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,
@@ -244,4 +245,18 @@ extern bool vfio_noiommu __read_mostly;
 enum { vfio_noiommu = false };
 #endif
 
+#ifdef CONFIG_HAVE_KVM
+void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm);
+void vfio_device_put_kvm(struct vfio_device *device);
+#else
+static inline void _vfio_device_get_kvm_safe(struct vfio_device *device,
+                                            struct kvm *kvm)
+{
+}
+
+static inline void vfio_device_put_kvm(struct vfio_device *device)
+{
+}
+#endif
+
 #endif

diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
index 5177bb0..28c47cd 100644 (file)
--- 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>
@@ -338,6 +341,55 @@ void vfio_unregister_group_dev(struct vfio_device *device)
 }
 EXPORT_SYMBOL_GPL(vfio_unregister_group_dev);
 
+#ifdef CONFIG_HAVE_KVM
+void _vfio_device_get_kvm_safe(struct vfio_device *device, struct kvm *kvm)
+{
+       void (*pfn)(struct kvm *kvm);
+       bool (*fn)(struct kvm *kvm);
+       bool ret;
+
+       lockdep_assert_held(&device->dev_set->lock);
+
+       pfn = symbol_get(kvm_put_kvm);
+       if (WARN_ON(!pfn))
+               return;
+
+       fn = symbol_get(kvm_get_kvm_safe);
+       if (WARN_ON(!fn)) {
+               symbol_put(kvm_put_kvm);
+               return;
+       }
+
+       ret = fn(kvm);
+       symbol_put(kvm_get_kvm_safe);
+       if (!ret) {
+               symbol_put(kvm_put_kvm);
+               return;
+       }
+
+       device->put_kvm = pfn;
+       device->kvm = kvm;
+}
+
+void vfio_device_put_kvm(struct vfio_device *device)
+{
+       lockdep_assert_held(&device->dev_set->lock);
+
+       if (!device->kvm)
+               return;
+
+       if (WARN_ON(!device->put_kvm))
+               goto clear;
+
+       device->put_kvm(device->kvm);
+       device->put_kvm = NULL;
+       symbol_put(kvm_put_kvm);
+
+clear:
+       device->kvm = NULL;
+}
+#endif
+
 /* true if the vfio_device has open_device() called but not close_device() */
 static bool vfio_assert_device_open(struct vfio_device *device)
 {
@@ -361,7 +413,6 @@ 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 +421,6 @@ 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 +437,6 @@ 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 +449,14 @@ 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 +464,12 @@ 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 35be78e..87ff862 100644 (file)
--- 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;