[Intel-gfx] [PATCH 1/2] drm: avoid races with modesetting rights
Desmond Cheong Zhi Xi
desmondcheongzx at gmail.com
Sun Aug 15 07:10:16 UTC 2021
On 13/8/21 11:49 pm, Daniel Vetter wrote:
> On Fri, Aug 13, 2021 at 04:54:49PM +0800, Desmond Cheong Zhi Xi wrote:
>> In drm_client_modeset.c and drm_fb_helper.c,
>> drm_master_internal_{acquire,release} are used to avoid races with DRM
>> userspace. These functions hold onto drm_device.master_mutex while
>> committing, and bail if there's already a master.
>>
>> However, ioctls can still race between themselves. A
>> time-of-check-to-time-of-use error can occur if an ioctl that changes
>> the modeset has its rights revoked after it validates its permissions,
>> but before it completes.
>>
>> There are three ioctls that can affect modesetting permissions:
>>
>> - DROP_MASTER ioctl removes rights for a master and its leases
>>
>> - REVOKE_LEASE ioctl revokes rights for a specific lease
>>
>> - SET_MASTER ioctl sets the device master if the master role hasn't
>> been acquired yet
>>
>> All these races can be avoided by introducing an SRCU that acts as a
>> barrier for ioctls that can change modesetting permissions. Processes
>> that perform modesetting should hold a read lock on the new
>> drm_device.master_barrier_srcu, and ioctls that change these
>> permissions should call synchronize_srcu before returning.
>>
>> This ensures that any process that might have seen old permissions are
>> flushed out before DROP_MASTER/REVOKE_LEASE/SET_MASTER ioctls return
>> to userspace.
>>
>> Reported-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
>
> This looks pretty solid, but I think there's one gap where we can still
> race. Scenario.
>
> Process A has a drm fd with master rights and two threads:
> - thread 1 does a long-running display operation (like a modeset or
> whatever)
> - thread 2 does a drop-master
>
> Then we start a new process B, which acquires master in drm_open (there is
> no other one left). This is like setmaster ioctl, but your
> DRM_MASTER_FLUSH bit doesn't work there.
>
Ah, I see the race. I think a good place to plug this would be in
drm_master_open using the drm_master_flush (or
drm_master_unlock_and_flush) that you suggested below.
> The other thing is that for modeset stuff (which this all is) srcu is
> probably massive overkill, and a simple rwsem should be good enough too.
> Maybe even better, since the rwsem guarantees that no new reader can start
> once you try to acquire the write side.
>
Makes sense, I'll switch to a rwsem then.
> Finally, and this is a bit a bikeshed: I don't like much how
> DRM_MASTER_FLUSH leaks the need of these very few places into the very
> core drm_ioctl function. One idea I had was to use task_work in a special
> function, roughly
>
> void master_flush()
> {
> down_write(master_rwsem);
> up_write(master_rwms);
> }
> void drm_master_flush()
> {
> init_task_work(fpriv->master_flush_work, master_flush)
> task_work_add(fpriv->master_flush_work);
> /* if task_work_add fails we're exiting, at which point the lack
> * of master flush doesn't matter);
> }
>
> And maybe put a comment above the function explaining why and how this
> works.
>
> We could even do a drm_master_unlock_and_flush helper, since that's really
> what everyone wants, and it would make it very clear which master state
> changes need this flush. Instead of setting a flag bit in an ioctl table
> very far away ...
>
> Thoughts?
> -Daniel
>
Sounds good. I wasn't aware of the task_work_add mechanism to queue work
before the task returns to usermode, but this seems like a more explicit
way to flush.
Thanks for the feedback, Daniel. I'll fix this up in a v2.
>> ---
>> drivers/gpu/drm/drm_auth.c | 17 ++++++++++++++---
>> drivers/gpu/drm/drm_client_modeset.c | 10 ++++++----
>> drivers/gpu/drm/drm_drv.c | 2 ++
>> drivers/gpu/drm/drm_fb_helper.c | 20 ++++++++++++--------
>> drivers/gpu/drm/drm_internal.h | 5 +++--
>> drivers/gpu/drm/drm_ioctl.c | 25 +++++++++++++++++++++----
>> include/drm/drm_device.h | 11 +++++++++++
>> include/drm/drm_ioctl.h | 7 +++++++
>> 8 files changed, 76 insertions(+), 21 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index 60a6b21474b1..004506608e76 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -29,6 +29,7 @@
>> */
>>
>> #include <linux/slab.h>
>> +#include <linux/srcu.h>
>>
>> #include <drm/drm_auth.h>
>> #include <drm/drm_drv.h>
>> @@ -448,21 +449,31 @@ void drm_master_put(struct drm_master **master)
>> EXPORT_SYMBOL(drm_master_put);
>>
>> /* Used by drm_client and drm_fb_helper */
>> -bool drm_master_internal_acquire(struct drm_device *dev)
>> +bool drm_master_internal_acquire(struct drm_device *dev, int *idx)
>> {
>> + *idx = srcu_read_lock(&dev->master_barrier_srcu);
>> +
>> mutex_lock(&dev->master_mutex);
>> if (dev->master) {
>> mutex_unlock(&dev->master_mutex);
>> + srcu_read_unlock(&dev->master_barrier_srcu, *idx);
>> return false;
>> }
>> + mutex_unlock(&dev->master_mutex);
>>
>> return true;
>> }
>> EXPORT_SYMBOL(drm_master_internal_acquire);
>>
>> /* Used by drm_client and drm_fb_helper */
>> -void drm_master_internal_release(struct drm_device *dev)
>> +void drm_master_internal_release(struct drm_device *dev, int idx)
>> {
>> - mutex_unlock(&dev->master_mutex);
>> + srcu_read_unlock(&dev->master_barrier_srcu, idx);
>> }
>> EXPORT_SYMBOL(drm_master_internal_release);
>> +
>> +/* Used by drm_ioctl */
>> +void drm_master_flush(struct drm_device *dev)
>> +{
>> + synchronize_srcu(&dev->master_barrier_srcu);
>> +}
>> diff --git a/drivers/gpu/drm/drm_client_modeset.c b/drivers/gpu/drm/drm_client_modeset.c
>> index ced09c7c06f9..9885f36f71b7 100644
>> --- a/drivers/gpu/drm/drm_client_modeset.c
>> +++ b/drivers/gpu/drm/drm_client_modeset.c
>> @@ -1165,13 +1165,14 @@ int drm_client_modeset_commit(struct drm_client_dev *client)
>> {
>> struct drm_device *dev = client->dev;
>> int ret;
>> + int idx;
>>
>> - if (!drm_master_internal_acquire(dev))
>> + if (!drm_master_internal_acquire(dev, &idx))
>> return -EBUSY;
>>
>> ret = drm_client_modeset_commit_locked(client);
>>
>> - drm_master_internal_release(dev);
>> + drm_master_internal_release(dev, idx);
>>
>> return ret;
>> }
>> @@ -1215,8 +1216,9 @@ int drm_client_modeset_dpms(struct drm_client_dev *client, int mode)
>> {
>> struct drm_device *dev = client->dev;
>> int ret = 0;
>> + int idx;
>>
>> - if (!drm_master_internal_acquire(dev))
>> + if (!drm_master_internal_acquire(dev, &idx))
>> return -EBUSY;
>>
>> mutex_lock(&client->modeset_mutex);
>> @@ -1226,7 +1228,7 @@ int drm_client_modeset_dpms(struct drm_client_dev *client, int mode)
>> drm_client_modeset_dpms_legacy(client, mode);
>> mutex_unlock(&client->modeset_mutex);
>>
>> - drm_master_internal_release(dev);
>> + drm_master_internal_release(dev, idx);
>>
>> return ret;
>> }
>> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
>> index 7a5097467ba5..c313f0674db3 100644
>> --- a/drivers/gpu/drm/drm_drv.c
>> +++ b/drivers/gpu/drm/drm_drv.c
>> @@ -574,6 +574,7 @@ static void drm_dev_init_release(struct drm_device *dev, void *res)
>> mutex_destroy(&dev->clientlist_mutex);
>> mutex_destroy(&dev->filelist_mutex);
>> mutex_destroy(&dev->struct_mutex);
>> + cleanup_srcu_struct(&dev->master_barrier_srcu);
>> drm_legacy_destroy_members(dev);
>> }
>>
>> @@ -612,6 +613,7 @@ static int drm_dev_init(struct drm_device *dev,
>> mutex_init(&dev->filelist_mutex);
>> mutex_init(&dev->clientlist_mutex);
>> mutex_init(&dev->master_mutex);
>> + init_srcu_struct(&dev->master_barrier_srcu);
>>
>> ret = drmm_add_action(dev, drm_dev_init_release, NULL);
>> if (ret)
>> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
>> index 3ab078321045..0d594bc15f18 100644
>> --- a/drivers/gpu/drm/drm_fb_helper.c
>> +++ b/drivers/gpu/drm/drm_fb_helper.c
>> @@ -1116,13 +1116,14 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
>> struct drm_fb_helper *fb_helper = info->par;
>> struct drm_device *dev = fb_helper->dev;
>> int ret;
>> + int idx;
>>
>> if (oops_in_progress)
>> return -EBUSY;
>>
>> mutex_lock(&fb_helper->lock);
>>
>> - if (!drm_master_internal_acquire(dev)) {
>> + if (!drm_master_internal_acquire(dev, &idx)) {
>> ret = -EBUSY;
>> goto unlock;
>> }
>> @@ -1136,7 +1137,7 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct fb_info *info)
>> ret = setcmap_legacy(cmap, info);
>> mutex_unlock(&fb_helper->client.modeset_mutex);
>>
>> - drm_master_internal_release(dev);
>> + drm_master_internal_release(dev, idx);
>> unlock:
>> mutex_unlock(&fb_helper->lock);
>>
>> @@ -1160,9 +1161,10 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
>> struct drm_device *dev = fb_helper->dev;
>> struct drm_crtc *crtc;
>> int ret = 0;
>> + int idx;
>>
>> mutex_lock(&fb_helper->lock);
>> - if (!drm_master_internal_acquire(dev)) {
>> + if (!drm_master_internal_acquire(dev, &idx)) {
>> ret = -EBUSY;
>> goto unlock;
>> }
>> @@ -1204,7 +1206,7 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
>> ret = -ENOTTY;
>> }
>>
>> - drm_master_internal_release(dev);
>> + drm_master_internal_release(dev, idx);
>> unlock:
>> mutex_unlock(&fb_helper->lock);
>> return ret;
>> @@ -1474,12 +1476,13 @@ int drm_fb_helper_pan_display(struct fb_var_screeninfo *var,
>> struct drm_fb_helper *fb_helper = info->par;
>> struct drm_device *dev = fb_helper->dev;
>> int ret;
>> + int idx;
>>
>> if (oops_in_progress)
>> return -EBUSY;
>>
>> mutex_lock(&fb_helper->lock);
>> - if (!drm_master_internal_acquire(dev)) {
>> + if (!drm_master_internal_acquire(dev, &idx)) {
>> ret = -EBUSY;
>> goto unlock;
>> }
>> @@ -1489,7 +1492,7 @@ int drm_fb_helper_pan_display(struct fb_var_screeninfo *var,
>> else
>> ret = pan_display_legacy(var, info);
>>
>> - drm_master_internal_release(dev);
>> + drm_master_internal_release(dev, idx);
>> unlock:
>> mutex_unlock(&fb_helper->lock);
>>
>> @@ -1948,6 +1951,7 @@ EXPORT_SYMBOL(drm_fb_helper_initial_config);
>> int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
>> {
>> int err = 0;
>> + int idx;
>>
>> if (!drm_fbdev_emulation || !fb_helper)
>> return 0;
>> @@ -1959,13 +1963,13 @@ int drm_fb_helper_hotplug_event(struct drm_fb_helper *fb_helper)
>> return err;
>> }
>>
>> - if (!fb_helper->fb || !drm_master_internal_acquire(fb_helper->dev)) {
>> + if (!fb_helper->fb || !drm_master_internal_acquire(fb_helper->dev, &idx)) {
>> fb_helper->delayed_hotplug = true;
>> mutex_unlock(&fb_helper->lock);
>> return err;
>> }
>>
>> - drm_master_internal_release(fb_helper->dev);
>> + drm_master_internal_release(fb_helper->dev, idx);
>>
>> drm_dbg_kms(fb_helper->dev, "\n");
>>
>> diff --git a/drivers/gpu/drm/drm_internal.h b/drivers/gpu/drm/drm_internal.h
>> index 17f3548c8ed2..578fd2769913 100644
>> --- a/drivers/gpu/drm/drm_internal.h
>> +++ b/drivers/gpu/drm/drm_internal.h
>> @@ -142,8 +142,9 @@ int drm_dropmaster_ioctl(struct drm_device *dev, void *data,
>> struct drm_file *file_priv);
>> int drm_master_open(struct drm_file *file_priv);
>> void drm_master_release(struct drm_file *file_priv);
>> -bool drm_master_internal_acquire(struct drm_device *dev);
>> -void drm_master_internal_release(struct drm_device *dev);
>> +bool drm_master_internal_acquire(struct drm_device *dev, int *idx);
>> +void drm_master_internal_release(struct drm_device *dev, int idx);
>> +void drm_master_flush(struct drm_device *dev);
>>
>> /* drm_sysfs.c */
>> extern struct class *drm_class;
>> diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
>> index be4a52dc4d6f..eb4ec3fab7d1 100644
>> --- a/drivers/gpu/drm/drm_ioctl.c
>> +++ b/drivers/gpu/drm/drm_ioctl.c
>> @@ -600,8 +600,10 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
>> DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_SET_SAREA_CTX, drm_legacy_setsareactx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
>> DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_GET_SAREA_CTX, drm_legacy_getsareactx, DRM_AUTH),
>>
>> - DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl, 0),
>> - DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl, 0),
>> + DRM_IOCTL_DEF(DRM_IOCTL_SET_MASTER, drm_setmaster_ioctl,
>> + DRM_MASTER_FLUSH),
>> + DRM_IOCTL_DEF(DRM_IOCTL_DROP_MASTER, drm_dropmaster_ioctl,
>> + DRM_MASTER_FLUSH),
>>
>> DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_ADD_CTX, drm_legacy_addctx, DRM_AUTH|DRM_ROOT_ONLY),
>> DRM_LEGACY_IOCTL_DEF(DRM_IOCTL_RM_CTX, drm_legacy_rmctx, DRM_AUTH|DRM_MASTER|DRM_ROOT_ONLY),
>> @@ -722,7 +724,8 @@ static const struct drm_ioctl_desc drm_ioctls[] = {
>> DRM_IOCTL_DEF(DRM_IOCTL_MODE_CREATE_LEASE, drm_mode_create_lease_ioctl, DRM_MASTER),
>> 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),
>> + DRM_IOCTL_DEF(DRM_IOCTL_MODE_REVOKE_LEASE, drm_mode_revoke_lease_ioctl,
>> + DRM_MASTER | DRM_MASTER_FLUSH),
>> };
>>
>> #define DRM_CORE_IOCTL_COUNT ARRAY_SIZE( drm_ioctls )
>> @@ -781,13 +784,17 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
>> struct drm_file *file_priv = file->private_data;
>> struct drm_device *dev = file_priv->minor->dev;
>> int retcode;
>> + int idx;
>>
>> if (drm_dev_is_unplugged(dev))
>> return -ENODEV;
>>
>> + if (unlikely(flags & DRM_MASTER))
>> + idx = srcu_read_lock(&dev->master_barrier_srcu);
>> +
>> retcode = drm_ioctl_permit(flags, file_priv);
>> if (unlikely(retcode))
>> - return retcode;
>> + goto release_master;
>>
>> /* Enforce sane locking for modern driver ioctls. */
>> if (likely(!drm_core_check_feature(dev, DRIVER_LEGACY)) ||
>> @@ -798,6 +805,16 @@ long drm_ioctl_kernel(struct file *file, drm_ioctl_t *func, void *kdata,
>> retcode = func(dev, kdata, file_priv);
>> mutex_unlock(&drm_global_mutex);
>> }
>> +
>> +release_master:
>> + if (unlikely(flags & DRM_MASTER))
>> + srcu_read_unlock(&dev->master_barrier_srcu, idx);
>> + /* After flushing, processes are guaranteed to see the new master/lease
>> + * permissions, and any process which might have seen the old
>> + * permissions is guaranteed to have finished.
>> + */
>> + if (unlikely(flags & DRM_MASTER_FLUSH))
>> + drm_master_flush(dev);
>> return retcode;
>> }
>> EXPORT_SYMBOL(drm_ioctl_kernel);
>> diff --git a/include/drm/drm_device.h b/include/drm/drm_device.h
>> index 604b1d1b2d72..0ac5fdb375f8 100644
>> --- a/include/drm/drm_device.h
>> +++ b/include/drm/drm_device.h
>> @@ -111,6 +111,17 @@ struct drm_device {
>> */
>> struct drm_master *master;
>>
>> + /**
>> + * @master_barrier_srcu:
>> + *
>> + * Used to synchronize modesetting rights between multiple users. Users
>> + * that can change the modeset or display state must hold an
>> + * srcu_read_lock() on @master_barrier_srcu, and ioctls that can change
>> + * modesetting rights should call synchronize_srcu() before returning
>> + * to userspace.
>> + */
>> + struct srcu_struct master_barrier_srcu;
>> +
>> /**
>> * @driver_features: per-device driver features
>> *
>> diff --git a/include/drm/drm_ioctl.h b/include/drm/drm_ioctl.h
>> index afb27cb6a7bd..13a68cdcea36 100644
>> --- a/include/drm/drm_ioctl.h
>> +++ b/include/drm/drm_ioctl.h
>> @@ -130,6 +130,13 @@ enum drm_ioctl_flags {
>> * not set DRM_AUTH because they do not require authentication.
>> */
>> DRM_RENDER_ALLOW = BIT(5),
>> + /**
>> + * @DRM_MASTER_FLUSH:
>> + *
>> + * This must be set for any ioctl which can change the modesetting
>> + * permissions for DRM users.
>> + */
>> + DRM_MASTER_FLUSH = BIT(6),
>> };
>>
>> /**
>> --
>> 2.25.1
>>
>
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