On Wed, Aug 18, 2021 at 03:38:22PM +0800, Desmond Cheong Zhi Xi wrote:

In a future patch, a read lock on drm_device.master_rwsem is held in the ioctl handler before the check for ioctl permissions. However, this produces the following lockdep splat:

====================================================== WARNING: possible circular locking dependency detected 5.14.0-rc6-CI-Patchwork_20831+ #1 Tainted: G U

---

kms_lease/1752 is trying to acquire lock: ffffffff827bad88 (drm_global_mutex){+.+.}-{3:3}, at: drm_open+0x64/0x280

but task is already holding lock: ffff88812e350108 (&dev->master_rwsem){++++}-{3:3}, at: drm_ioctl_kernel+0xfb/0x1a0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (&dev->master_rwsem){++++}-{3:3}: lock_acquire+0xd3/0x310 down_read+0x3b/0x140 drm_master_internal_acquire+0x1d/0x60 drm_client_modeset_commit+0x10/0x40 __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+0x1de/0x2c0 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+0x37/0x390 kthread+0x13b/0x170 ret_from_fork+0x1f/0x30

-> #1 (&helper->lock){+.+.}-{3:3}: lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 __drm_fb_helper_restore_fbdev_mode_unlocked+0x44/0xb0 intel_fbdev_restore_mode+0x2b/0x50 [i915] drm_lastclose+0x27/0x50 drm_release_noglobal+0x42/0x60 __fput+0x9e/0x250 task_work_run+0x6b/0xb0 exit_to_user_mode_prepare+0x1c5/0x1d0 syscall_exit_to_user_mode+0x19/0x50 do_syscall_64+0x46/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae

-> #0 (drm_global_mutex){+.+.}-{3:3}: validate_chain+0xb39/0x1e70 __lock_acquire+0x5a1/0xb70 lock_acquire+0xd3/0x310 __mutex_lock+0xa8/0x930 drm_open+0x64/0x280 drm_stub_open+0x9f/0x100 chrdev_open+0x9f/0x1d0 do_dentry_open+0x14a/0x3a0 dentry_open+0x53/0x70 drm_mode_create_lease_ioctl+0x3cb/0x970 drm_ioctl_kernel+0xc9/0x1a0 drm_ioctl+0x201/0x3d0 __x64_sys_ioctl+0x6a/0xa0 do_syscall_64+0x37/0xb0 entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this: Chain exists of: drm_global_mutex --> &helper->lock --> &dev->master_rwsem Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(&dev->master_rwsem); lock(&helper->lock); lock(&dev->master_rwsem); lock(drm_global_mutex);

*** DEADLOCK ***

The lock hierarchy inversion happens because we grab the drm_global_mutex while already holding on to master_rwsem. To avoid this, we do some prep work to grab the drm_global_mutex before checking for ioctl permissions.

At the same time, we update the check for the global mutex to use the drm_dev_needs_global_mutex helper function.

This is intentional, essentially we force all non-legacy drivers to have unlocked ioctl (otherwise everyone forgets to set that flag).

For non-legacy drivers the global lock only ensures ordering between drm_open and lastclose (I think at least), and between drm_dev_register/unregister and the backwards ->load/unload callbacks (which are called in the wrong place, but we cannot fix that for legacy drivers).

->load/unload should be completely unused (maybe radeon still uses it), and ->lastclose is also on the decline.

Maybe we should update the comment of drm_global_mutex to explain what it protects and why.

I'm also confused how this patch connects to the splat, since for i915 we shouldn't be taking the drm_global_lock here at all. The problem seems to be the drm_open_helper when we create a new lease, which is an entirely different can of worms.

I'm honestly not sure how to best do that, but we should be able to create a file and then call drm_open_helper directly, or well a version of that which never takes the drm_global_mutex. Because that is not needed for nested drm_file opening: - legacy drivers never go down this path because leases are only supported with modesetting, and modesetting is only supported for non-legacy drivers - the races against dev->open_count due to last_close or ->load callbacks don't matter, because for the entire ioctl we already have an open drm_file and that wont disappear.

So this should work, but I'm not entirely sure how to make it work. -Daniel

Signed-off-by: Desmond Cheong Zhi Xi desmondcheongzx@gmail.com

drivers/gpu/drm/drm_ioctl.c | 18 +++++++++--------- 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c index 880fc565d599..2cb57378a787 100644 --- a/drivers/gpu/drm/drm_ioctl.c +++ b/drivers/gpu/drm/drm_ioctl.c @@ -779,19 +779,19 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata, if (drm_dev_is_unplugged(dev)) return -ENODEV;

• /* Enforce sane locking for modern driver ioctls. */
• if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED))

• mutex_lock(&drm_global_mutex);
  

• retcode = drm_ioctl_permit(flags, file_priv); if (unlikely(retcode))

• return retcode;
  

• goto out;
  

• /* Enforce sane locking for modern driver ioctls. */
• if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||

•    (flags & DRM_UNLOCKED))
  

• retcode = func(dev, kdata, file_priv);
  

• else {

• mutex_lock(&drm_global_mutex);
  

• retcode = func(dev, kdata, file_priv);
  

• retcode = func(dev, kdata, file_priv);

+out:

• if (unlikely(drm_dev_needs_global_mutex(dev)) && !(flags & DRM_UNLOCKED)) mutex_unlock(&drm_global_mutex);

• } return retcode;

} EXPORT_SYMBOL(drm_ioctl_kernel); -- 2.25.1