On 23/6/21 7:37 pm, Desmond Cheong Zhi Xi wrote:
While checking the master status of the DRM file in drm_is_current_master(), the device's master mutex should be held. Without the mutex, the pointer fpriv->master may be freed concurrently by another process calling drm_setmaster_ioctl(). This could lead to use-after-free errors when the pointer is subsequently dereferenced in drm_lease_owner().
The callers of drm_is_current_master() from drm_auth.c hold the device's master mutex, but external callers do not. Hence, we implement drm_is_current_master_locked() to be used within drm_auth.c, and modify drm_is_current_master() to grab the device's master mutex before checking the master status.
Additionally, to avoid a circular lock dependency by introducing drm_device.master_mutex into drm_is_current_master(), we move the call to drm_is_current_master() in drm_mode_getconnector out from the section locked by &dev->mode_config.mutex.
Failing to avoid this lock dependency produces the following lockdep splat:
====================================================== WARNING: possible circular locking dependency detected 5.13.0-rc7-CI-CI_DRM_10254+ #1 Not tainted
kms_frontbuffer/1087 is trying to acquire lock: ffff88810dcd01a8 (&dev->master_mutex){+.+.}-{3:3}, at: drm_is_current_master+0x1b/0x40 but task is already holding lock: ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0 which lock already depends on the new lock. the existing dependency chain (in reverse order) is: -> #2 (&dev->mode_config.mutex){+.+.}-{3:3}: __mutex_lock+0xab/0x970 drm_client_modeset_probe+0x22e/0xca0 __drm_fb_helper_initial_config_and_unlock+0x42/0x540 intel_fbdev_initial_config+0xf/0x20 [i915] async_run_entry_fn+0x28/0x130 process_one_work+0x26d/0x5c0 worker_thread+0x37/0x380 kthread+0x144/0x170 ret_from_fork+0x1f/0x30 -> #1 (&client->modeset_mutex){+.+.}-{3:3}: __mutex_lock+0xab/0x970 drm_client_modeset_commit_locked+0x1c/0x180 drm_client_modeset_commit+0x1c/0x40 __drm_fb_helper_restore_fbdev_mode_unlocked+0x88/0xb0 drm_fb_helper_set_par+0x34/0x40 intel_fbdev_set_par+0x11/0x40 [i915] fbcon_init+0x270/0x4f0 visual_init+0xc6/0x130 do_bind_con_driver+0x1e5/0x2d0 do_take_over_console+0x10e/0x180 do_fbcon_takeover+0x53/0xb0 register_framebuffer+0x22d/0x310 __drm_fb_helper_initial_config_and_unlock+0x36c/0x540 intel_fbdev_initial_config+0xf/0x20 [i915] async_run_entry_fn+0x28/0x130 process_one_work+0x26d/0x5c0 worker_thread+0x37/0x380 kthread+0x144/0x170 ret_from_fork+0x1f/0x30 -> #0 (&dev->master_mutex){+.+.}-{3:3}: __lock_acquire+0x151e/0x2590 lock_acquire+0xd1/0x3d0 __mutex_lock+0xab/0x970 drm_is_current_master+0x1b/0x40 drm_mode_getconnector+0x37e/0x4a0 drm_ioctl_kernel+0xa8/0xf0 drm_ioctl+0x1e8/0x390 __x64_sys_ioctl+0x6a/0xa0 do_syscall_64+0x39/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae other info that might help us debug this: Chain exists of: &dev->master_mutex --> &client->modeset_mutex --> &dev->mode_config.mutex Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&dev->mode_config.mutex); lock(&client->modeset_mutex); lock(&dev->mode_config.mutex); lock(&dev->master_mutex); *** DEADLOCK *** 1 lock held by kms_frontbuffer/1087: #0: ffff88810dcd0488 (&dev->mode_config.mutex){+.+.}-{3:3}, at: drm_mode_getconnector+0x1c6/0x4a0 stack backtrace: CPU: 7 PID: 1087 Comm: kms_frontbuffer Not tainted 5.13.0-rc7-CI-CI_DRM_10254+ #1 Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.3234.A01.1906141750 06/14/2019 Call Trace: dump_stack+0x7f/0xad check_noncircular+0x12e/0x150 __lock_acquire+0x151e/0x2590 lock_acquire+0xd1/0x3d0 __mutex_lock+0xab/0x970 drm_is_current_master+0x1b/0x40 drm_mode_getconnector+0x37e/0x4a0 drm_ioctl_kernel+0xa8/0xf0 drm_ioctl+0x1e8/0x390 __x64_sys_ioctl+0x6a/0xa0 do_syscall_64+0x39/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae
Reported-by: Daniel Vetter daniel.vetter@ffwll.ch Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com
drivers/gpu/drm/drm_auth.c | 51 +++++++++++++++++++++------------ drivers/gpu/drm/drm_connector.c | 5 +++- 2 files changed, 36 insertions(+), 20 deletions(-)
diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c index f00e5abdbbf4..ab1863c5a5a0 100644 --- a/drivers/gpu/drm/drm_auth.c +++ b/drivers/gpu/drm/drm_auth.c @@ -61,6 +61,35 @@
- trusted clients.
*/
+static bool drm_is_current_master_locked(struct drm_file *fpriv) +{
- lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
- return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
+}
+/**
- drm_is_current_master - checks whether @priv is the current master
- @fpriv: DRM file private
- Checks whether @fpriv is current master on its device. This decides whether a
- client is allowed to run DRM_MASTER IOCTLs.
- Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
- the current master is assumed to own the non-shareable display hardware.
- */
+bool drm_is_current_master(struct drm_file *fpriv) +{
- bool ret;
- mutex_lock(&fpriv->minor->dev->master_mutex);
- ret = drm_is_current_master_locked(fpriv);
- mutex_unlock(&fpriv->minor->dev->master_mutex);
- return ret;
+} +EXPORT_SYMBOL(drm_is_current_master);
- int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv) { struct drm_auth *auth = data;
@@ -223,7 +252,7 @@ int drm_setmaster_ioctl(struct drm_device *dev, void *data, if (ret) goto out_unlock;
- if (drm_is_current_master(file_priv))
if (drm_is_current_master_locked(file_priv)) goto out_unlock;
if (dev->master) {
@@ -272,7 +301,7 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data, if (ret) goto out_unlock;
- if (!drm_is_current_master(file_priv)) {
- if (!drm_is_current_master_locked(file_priv)) { ret = -EINVAL; goto out_unlock; }
@@ -321,7 +350,7 @@ void drm_master_release(struct drm_file *file_priv) if (file_priv->magic) idr_remove(&file_priv->master->magic_map, file_priv->magic);
- if (!drm_is_current_master(file_priv))
if (!drm_is_current_master_locked(file_priv)) goto out;
drm_legacy_lock_master_cleanup(dev, master);
@@ -342,22 +371,6 @@ void drm_master_release(struct drm_file *file_priv) mutex_unlock(&dev->master_mutex); }
-/**
- drm_is_current_master - checks whether @priv is the current master
- @fpriv: DRM file private
- Checks whether @fpriv is current master on its device. This decides whether a
- client is allowed to run DRM_MASTER IOCTLs.
- Most of the modern IOCTL which require DRM_MASTER are for kernel modesetting
- the current master is assumed to own the non-shareable display hardware.
- */
-bool drm_is_current_master(struct drm_file *fpriv) -{
- return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
-} -EXPORT_SYMBOL(drm_is_current_master);
- /**
- drm_master_get - reference a master pointer
- @master: &struct drm_master
diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c index da39e7ff6965..a5f63bad3893 100644 --- a/drivers/gpu/drm/drm_connector.c +++ b/drivers/gpu/drm/drm_connector.c @@ -2414,6 +2414,7 @@ int drm_mode_getconnector(struct drm_device *dev, void *data, struct drm_mode_modeinfo u_mode; struct drm_mode_modeinfo __user *mode_ptr; uint32_t __user *encoder_ptr;
bool is_current_master;
if (!drm_core_check_feature(dev, DRIVER_MODESET)) return -EOPNOTSUPP;
@@ -2444,9 +2445,11 @@ int drm_mode_getconnector(struct drm_device *dev, void *data, out_resp->connector_type = connector->connector_type; out_resp->connector_type_id = connector->connector_type_id;
- is_current_master = drm_is_current_master(file_priv)
- mutex_lock(&dev->mode_config.mutex); if (out_resp->count_modes == 0) {
if (drm_is_current_master(file_priv))
if (is_current_master) connector->funcs->fill_modes(connector, dev->mode_config.max_width, dev->mode_config.max_height);
Hi Daniel,
My bad, I just realized that this patch won't apply to stable because of the changes to drm_connector.c.
Will the intel-gfx CI apply the patches in order if I break up this patch and move the changes to drm_connector.c earlier in the series?
Best wishes, Desmond