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SubjectRe: [PATCH] drm: Prevent modeset helpers to access an uninitialized drm_mode_config
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Hi Javier

Am 24.07.22 um 14:37 schrieb Javier Martinez Canillas:
> DRM drivers initialize the mode configuration with drmm_mode_config_init()
> and that function (among other things) initializes mutexes that are later
> used by modeset helpers.
>
> But the helpers should only attempt to grab those locks if the mode config
> was properly initialized. Otherwise it can lead to kernel oops. An example
> is when a DRM driver using the component framework does not initialize the
> drm_mode_config, because its .bind callback was not being executed due one
> of its expected sub-devices' driver failing to probe.
>
> Some drivers check the struct drm_driver.registered field as an indication
> on whether their .shutdown callback should call helpers to tearn down the
> mode configuration or not, but most drivers just assume that it is always
> safe to call helpers such as drm_atomic_helper_shutdown() during shutdown.
>
> Let make the DRM core more robust and prevent this to happen, by marking a
> struct drm_mode_config as initialized during drmm_mode_config_init(). that
> way helpers can check for it and not attempt to grab uninitialized mutexes.

I disagree. This patch looks like cargo-cult programming and entirely
arbitrary. The solution here is to fix drivers. The actual test to
perform is to instrument the mutex implementation to detect
uninitialized mutexes.

Best regards
Thomas

>
> Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
> ---
>
> drivers/gpu/drm/drm_mode_config.c | 4 ++++
> drivers/gpu/drm/drm_modeset_lock.c | 6 ++++++
> include/drm/drm_mode_config.h | 8 ++++++++
> 3 files changed, 18 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 59b34f07cfce..db649f97120b 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -456,6 +456,8 @@ int drmm_mode_config_init(struct drm_device *dev)
> dma_resv_fini(&resv);
> }
>
> + dev->mode_config.initialized = true;
> +
> return drmm_add_action_or_reset(dev, drm_mode_config_init_release,
> NULL);
> }
> @@ -549,6 +551,8 @@ void drm_mode_config_cleanup(struct drm_device *dev)
> idr_destroy(&dev->mode_config.tile_idr);
> idr_destroy(&dev->mode_config.object_idr);
> drm_modeset_lock_fini(&dev->mode_config.connection_mutex);
> +
> + dev->mode_config.initialized = false;
> }
> EXPORT_SYMBOL(drm_mode_config_cleanup);
>
> diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
> index 918065982db4..d6a81cb88123 100644
> --- a/drivers/gpu/drm/drm_modeset_lock.c
> +++ b/drivers/gpu/drm/drm_modeset_lock.c
> @@ -444,6 +444,9 @@ EXPORT_SYMBOL(drm_modeset_unlock);
> *
> * See also: DRM_MODESET_LOCK_ALL_BEGIN() and DRM_MODESET_LOCK_ALL_END()
> *
> + * This function must only be called after drmm_mode_config_init(), since it
> + * takes locks that are initialized as part of the initial mode configuration.
> + *
> * Returns: 0 on success or a negative error-code on failure.
> */
> int drm_modeset_lock_all_ctx(struct drm_device *dev,
> @@ -454,6 +457,9 @@ int drm_modeset_lock_all_ctx(struct drm_device *dev,
> struct drm_plane *plane;
> int ret;
>
> + if (WARN_ON(!dev->mode_config.initialized))
> + return -EINVAL;
> +
> ret = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
> if (ret)
> return ret;
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index 6b5e01295348..d2e1a6d7dcc2 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -954,6 +954,14 @@ struct drm_mode_config {
> struct drm_atomic_state *suspend_state;
>
> const struct drm_mode_config_helper_funcs *helper_private;
> +
> + /**
> + * @initialized:
> + *
> + * Internally used by modeset helpers such as drm_modeset_lock_all_ctx()
> + * to determine if the mode configuration has been properly initialized.
> + */
> + bool initialized;
> };
>
> int __must_check drmm_mode_config_init(struct drm_device *dev);

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev
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