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    Subject[PATCH AUTOSEL 5.14 035/252] drm: serialize drm_file.master with a new spinlock
    Date
    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 b59b26a71ad5..cbb896b91d94 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;
    @@ -303,10 +305,13 @@ 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 d4f0bac6f8f8..ceb1a9723855 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
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