diff options
author | Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com> | 2021-07-12 12:35:07 +0800 |
---|---|---|
committer | Daniel Vetter <daniel.vetter@ffwll.ch> | 2021-07-20 20:17:58 +0200 |
commit | 0b0860a3cf5eccf183760b1177a1dcdb821b0b66 (patch) | |
tree | d8893b9878ac83636e71fc5113443a8219d4968a /include | |
parent | 1f7ef07cfa14fb8557d1f1b7a14c76926142a4fb (diff) | |
download | linux-0b0860a3cf5eccf183760b1177a1dcdb821b0b66.tar.gz linux-0b0860a3cf5eccf183760b1177a1dcdb821b0b66.tar.bz2 linux-0b0860a3cf5eccf183760b1177a1dcdb821b0b66.zip |
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
Diffstat (limited to 'include')
-rw-r--r-- | include/drm/drm_file.h | 12 |
1 files changed, 9 insertions, 3 deletions
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; |