Re: [PATCH v3 7/9] drm: update global mutex lock in the ioctl handler
From: Desmond Cheong Zhi Xi
Date: Thu Aug 19 2021 - 06:53:09 EST
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:
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.
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
---
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