Re: [PATCH v3 7/9] drm: update global mutex lock in the ioctl handler

On 18/8/21 7:02 pm, Daniel Vetter wrote:

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

quoted

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.

Ah ok got it, I'll change the check back to
drm_core_check_feature(dev, DRIVER_LEGACY) then.

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

The comments in drm_dev_needs_global_mutex make sense I think, I just
didn't read the code closely enough.

I'm also confused how this patch connects to the splat, since for i915 we

Right, my bad, this is a separate instance of circular locking. I was
too hasty when I saw that for legacy drivers we might grab master_rwsem
then drm_global_mutex in the ioctl handler.

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

One idea that comes to mind is to change the outcome of
drm_dev_needs_global_mutex while we're in the ioctl, but that requires
more locking which sounds like a bad idea.

Another idea, which is quite messy, but just for thoughts, uses the idea
of pushing the master_rwsem read lock down:

diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
index 7f523e1c5650..5d05e744b728 100644
--- a/drivers/gpu/drm/drm_ioctl.c
+++ b/drivers/gpu/drm/drm_ioctl.c

@@ -712,7 +712,7 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
  		      DRM_RENDER_ALLOW),
  	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_GET_SEQUENCE, drm_crtc_get_sequence_ioctl, 0),
  	DRM_IOCTL_DEF(DRM_IOCTL_CRTC_QUEUE_SEQUENCE, drm_crtc_queue_sequence_ioctl, 0),
-	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER),
+	DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, 0),
  	DRM_IOCTL_DEF(DRM_IOCTL_MODE_LIST_LESSEES, drm_mode_list_lessees_ioctl, DRM_MASTER),
  	DRM_IOCTL_DEF(DRM_IOCTL_MODE_GET_LEASE, drm_mode_get_lease_ioctl, DRM_MASTER),
  	DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl, DRM_MASTER),

diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
index 983701198ffd..a25bc69522b4 100644
--- a/drivers/gpu/drm/drm_lease.c
+++ b/drivers/gpu/drm/drm_lease.c

@@ -500,6 +500,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  		return -EINVAL;
  	}
  
+	/* Clone the lessor file to create a new file for us */
+	DRM_DEBUG_LEASE("Allocating lease file\n");
+	lessee_file = file_clone_open(lessor_file);
+	if (IS_ERR(lessee_file))
+		return PTR_ERR(lessee_file);
+
+	down_read(&dev->master_rwsem);
+
+	if (!drm_is_current_master(lessor_priv)) {
+		ret = -EACCES;
+		goto out_file;
+	}
+
  	lessor = drm_file_get_master(lessor_priv);
  	/* Do not allow sub-leases */
  	if (lessor->lessor) {

@@ -547,14 +560,6 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  		goto out_leases;
  	}
  
-	/* Clone the lessor file to create a new file for us */
-	DRM_DEBUG_LEASE("Allocating lease file\n");
-	lessee_file = file_clone_open(lessor_file);
-	if (IS_ERR(lessee_file)) {
-		ret = PTR_ERR(lessee_file);
-		goto out_lessee;
-	}
-
  	lessee_priv = lessee_file->private_data;
  	/* Change the file to a master one */
  	drm_master_put(&lessee_priv->master);

@@ -571,17 +576,19 @@ int drm_mode_create_lease_ioctl(struct drm_device *dev,
  	fd_install(fd, lessee_file);
  
  	drm_master_put(&lessor);
+	up_read(&dev->master_rwsem);
  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl succeeded\n");
  	return 0;
  
-out_lessee:
-	drm_master_put(&lessee);
-
  out_leases:
  	put_unused_fd(fd);
  
  out_lessor:
  	drm_master_put(&lessor);
+
+out_file:
+	up_read(&dev->master_rwsem);
+	fput(lessee_file);
  	DRM_DEBUG_LEASE("drm_mode_create_lease_ioctl failed: %d\n", ret);
  	return ret;
  }

Something like this would also address the other deadlock we'd hit in
drm_mode_create_lease_ioctl():

   drm_ioctl_kernel():
     down_read(&master_rwsem);               <--- down_read()
     drm_mode_create_lease_ioctl():
       drm_lease_create():
         file_clone_open():
           ...
           drm_open():
             drm_open_helper():
               drm_master_open():
                 down_write(&master_rwsem);  <--- down_write()

Overall, I think the suggestion to push master_rwsem write locks down
into ioctls would solve the nesting problem for those ioctls.

Although I'm still a little concerned that, just like here, there might
be deeply embedded nested locking, so locking becomes prone to breaking.
It does smell a bit to me.

quoted

Signed-off-by: Desmond Cheong Zhi Xi <redacted>
---
  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