[PATCH AUTOSEL 5.13 028/219] drm: serialize drm_file.master with a new spinlock

Sasha Levin
2021-09-09T12:16:29Z

From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

[ Upstream commit 0b0860a3cf5eccf183760b1177a1dcdb821b0b66 ]

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
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/drm_auth.c | 17 +++++++++++------
drivers/gpu/drm/drm_file.c | 1 +
include/drm/drm_file.h | 12 +++++++++---
3 files changed, 21 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 232abbba3686..0024ad93d24b 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -135,16 +135,18 @@ 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;
@@ -302,10 +304,13 @@ int drm_master_open(struct drm_file *file_priv)
/* if there is no current master make this fd it, but do not create
* 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 7efbccffc2ea..c6feeb5651b0 100644
--- 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 b81b3bfb08c8..9b82988e3427 100644
--- 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;

--
2.30.2