drm: serialize drm_file.master with a new spinlock

Currently, drm_file.master pointers should be protected by
drm_device.master_mutex when being dereferenced. This is because
drm_file.master is not invariant for the lifetime of drm_file. If
drm_file is not the creator of master, then drm_file.is_master is
false, and a call to drm_setmaster_ioctl will invoke
drm_new_set_master, which then allocates a new master for drm_file and
puts the old master.

Thus, without holding drm_device.master_mutex, the old value of
drm_file.master could be freed while it is being used by another
concurrent process.

However, it is not always possible to lock drm_device.master_mutex to
dereference drm_file.master. Through the fbdev emulation code, this
might occur in a deep nest of other locks. But drm_device.master_mutex
is also the outermost lock in the nesting hierarchy, so this leads to
potential deadlocks.

To address this, we introduce a new spin lock at the bottom of the
lock hierarchy that only serializes drm_file.master. With this change,
the value of drm_file.master changes only when both
drm_device.master_mutex and drm_file.master_lookup_lock are
held. Hence, any process holding either of those locks can ensure that
the value of drm_file.master will not change concurrently.

Since no lock depends on the new drm_file.master_lookup_lock, when
drm_file.master is dereferenced, but drm_device.master_mutex cannot be
held, we can safely protect the master pointer with
drm_file.master_lookup_lock.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210712043508.11584-5-desmondcheongzx@gmail.com

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index ab1863c..30a2399 100644 (file)
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -164,16 +164,18 @@ static void drm_set_master(struct drm_device *dev, struct drm_file *fpriv,
 static int drm_new_set_master(struct drm_device *dev, struct drm_file *fpriv)
 {
        struct drm_master *old_master;
+       struct drm_master *new_master;
 
        lockdep_assert_held_once(&dev->master_mutex);
 
        WARN_ON(fpriv->is_master);
        old_master = fpriv->master;
-       fpriv->master = drm_master_create(dev);
-       if (!fpriv->master) {
-               fpriv->master = old_master;
+       new_master = drm_master_create(dev);
+       if (!new_master)
                return -ENOMEM;
-       }
+       spin_lock(&fpriv->master_lookup_lock);
+       fpriv->master = new_master;
+       spin_unlock(&fpriv->master_lookup_lock);
 
        fpriv->is_master = 1;
        fpriv->authenticated = 1;
@@ -332,10 +334,13 @@ int drm_master_open(struct drm_file *file_priv)
         * any master object for render clients
         */
        mutex_lock(&dev->master_mutex);
-       if (!dev->master)
+       if (!dev->master) {
                ret = drm_new_set_master(dev, file_priv);
-       else
+       } else {
+               spin_lock(&file_priv->master_lookup_lock);
                file_priv->master = drm_master_get(dev->master);
+               spin_unlock(&file_priv->master_lookup_lock);
+       }
        mutex_unlock(&dev->master_mutex);
 
        return ret;

diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
index d4f0bac..ceb1a97 100644 (file)
--- a/drivers/gpu/drm/drm_file.c
+++ b/drivers/gpu/drm/drm_file.c
@@ -176,6 +176,7 @@ struct drm_file *drm_file_alloc(struct drm_minor *minor)
        init_waitqueue_head(&file->event_wait);
        file->event_space = 4096; /* set aside 4k for event buffer */
 
+       spin_lock_init(&file->master_lookup_lock);
        mutex_init(&file->event_read_lock);
 
        if (drm_core_check_feature(dev, DRIVER_GEM))

diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index b81b3bf..9b82988 100644 (file)
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -226,15 +226,21 @@ struct drm_file {
        /**
         * @master:
         *
-        * Master this node is currently associated with. Only relevant if
-        * drm_is_primary_client() returns true. Note that this only
-        * matches &drm_device.master if the master is the currently active one.
+        * Master this node is currently associated with. Protected by struct
+        * &drm_device.master_mutex, and serialized by @master_lookup_lock.
+        *
+        * Only relevant if drm_is_primary_client() returns true. Note that
+        * this only matches &drm_device.master if the master is the currently
+        * active one.
         *
         * See also @authentication and @is_master and the :ref:`section on
         * primary nodes and authentication <drm_primary_node>`.
         */
        struct drm_master *master;
 
+       /** @master_lock: Serializes @master. */
+       spinlock_t master_lookup_lock;
+
        /** @pid: Process that opened this file. */
        struct pid *pid;