Re: [PATCH v4] drm/amdgpu/userq: refcount userqueues to avoid any race conditions

["Christian König" <christian.koenig@amd.com>]

On 3/3/26 13:06, Sunil Khatri wrote:
> To avoid race condition and avoid UAF cases, implement kref
> based queues and protect the below operations using xa lock
> a. Getting a queue from xarray
> b. Increment/Decrement it's refcount
> 
> Every time some one want to access a queue, always get via
> amdgpu_userq_get to make sure we have locks in place and get
> the object if active.
> 
> A userqueue is destroyed on the last refcount is dropped which
> typically would be via IOCTL or during fini.
> 
> v2: Add the missing drop in one the condition in the signal ioclt [Alex]
> 
> v3: remove the queue from the xarray first in the free queue ioctl path
>     [Christian]
> 
> - Pass queue to the amdgpu_userq_put directly.
> - make amdgpu_userq_put xa_lock free since we are doing put for each get
>   only and final put is done via destroy and we remove the queue from xa
>   with lock.
> - use userq_put in fini too so cleanup is done fully.
> 
> v4: Use xa_erase directly rather than doing load and erase in free
>     ioctl. Also remove some of the error logs which could be exploited
>     by the user to flood the logs [Christian]
> 
> Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
> ---
>  drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c     | 110 ++++++++++++------
>  drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h     |   4 +
>  .../gpu/drm/amd/amdgpu/amdgpu_userq_fence.c   |  14 ++-
>  3 files changed, 91 insertions(+), 37 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> index e07b2082cf25..ed6a9d779d1b 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.c
> @@ -446,8 +446,7 @@ static int amdgpu_userq_wait_for_last_fence(struct amdgpu_usermode_queue *queue)
>  	return ret;
>  }
>  
> -static void amdgpu_userq_cleanup(struct amdgpu_usermode_queue *queue,
> -				 u32 queue_id)
> +static void amdgpu_userq_cleanup(struct amdgpu_usermode_queue *queue)
>  {
>  	struct amdgpu_userq_mgr *uq_mgr = queue->userq_mgr;
>  	struct amdgpu_device *adev = uq_mgr->adev;
> @@ -461,7 +460,6 @@ static void amdgpu_userq_cleanup(struct amdgpu_usermode_queue *queue,
>  	uq_funcs->mqd_destroy(queue);
>  	amdgpu_userq_fence_driver_free(queue);
>  	/* Use interrupt-safe locking since IRQ handlers may access these XArrays */
> -	xa_erase_irq(&uq_mgr->userq_xa, queue_id);
>  	xa_erase_irq(&adev->userq_doorbell_xa, queue->doorbell_index);
>  	queue->userq_mgr = NULL;
>  	list_del(&queue->userq_va_list);
> @@ -470,12 +468,6 @@ static void amdgpu_userq_cleanup(struct amdgpu_usermode_queue *queue,
>  	up_read(&adev->reset_domain->sem);
>  }
>  
> -static struct amdgpu_usermode_queue *
> -amdgpu_userq_find(struct amdgpu_userq_mgr *uq_mgr, u32 qid)
> -{
> -	return xa_load(&uq_mgr->userq_xa, qid);
> -}
> -
>  void
>  amdgpu_userq_ensure_ev_fence(struct amdgpu_userq_mgr *uq_mgr,
>  			     struct amdgpu_eviction_fence_mgr *evf_mgr)
> @@ -625,22 +617,13 @@ amdgpu_userq_get_doorbell_index(struct amdgpu_userq_mgr *uq_mgr,
>  }
>  
>  static int
> -amdgpu_userq_destroy(struct drm_file *filp, u32 queue_id)
> +amdgpu_userq_destroy(struct amdgpu_userq_mgr *uq_mgr, struct amdgpu_usermode_queue *queue)
>  {
> -	struct amdgpu_fpriv *fpriv = filp->driver_priv;
> -	struct amdgpu_userq_mgr *uq_mgr = &fpriv->userq_mgr;
>  	struct amdgpu_device *adev = uq_mgr->adev;
> -	struct amdgpu_usermode_queue *queue;
>  	int r = 0;
>  
>  	cancel_delayed_work_sync(&uq_mgr->resume_work);
>  	mutex_lock(&uq_mgr->userq_mutex);
> -	queue = amdgpu_userq_find(uq_mgr, queue_id);
> -	if (!queue) {
> -		drm_dbg_driver(adev_to_drm(uq_mgr->adev), "Invalid queue id to destroy\n");
> -		mutex_unlock(&uq_mgr->userq_mutex);
> -		return -EINVAL;
> -	}
>  	amdgpu_userq_wait_for_last_fence(queue);
>  	/* Cancel any pending hang detection work and cleanup */
>  	if (queue->hang_detect_fence) {
> @@ -672,13 +655,44 @@ amdgpu_userq_destroy(struct drm_file *filp, u32 queue_id)
>  		drm_warn(adev_to_drm(uq_mgr->adev), "trying to destroy a HW mapping userq\n");
>  		queue->state = AMDGPU_USERQ_STATE_HUNG;
>  	}
> -	amdgpu_userq_cleanup(queue, queue_id);
> +	amdgpu_userq_cleanup(queue);
>  	mutex_unlock(&uq_mgr->userq_mutex);
>  
>  	pm_runtime_put_autosuspend(adev_to_drm(adev)->dev);
>  
>  	return r;
>  }
> +static void amdgpu_userq_kref_destroy(struct kref *kref)
> +{
> +	int r;
> +	struct amdgpu_usermode_queue *queue =
> +		container_of(kref, struct amdgpu_usermode_queue, refcount);
> +
> +	struct amdgpu_userq_mgr *uq_mgr = queue->userq_mgr;
> +
> +	r = amdgpu_userq_destroy(uq_mgr, queue);
> +	if (r)
> +		drm_file_err(uq_mgr->file, "Failed to destroy usermode queue\n");
> +}
> +
> +struct amdgpu_usermode_queue *amdgpu_userq_get(struct amdgpu_userq_mgr *uq_mgr, u32 qid)
> +{
> +	struct amdgpu_usermode_queue *queue;
> +
> +	xa_lock(&uq_mgr->userq_xa);
> +	queue = xa_load(&uq_mgr->userq_xa, qid);
> +	if (queue)
> +		kref_get(&queue->refcount);
> +	xa_unlock(&uq_mgr->userq_xa);
> +
> +	return queue;
> +}
> +
> +void amdgpu_userq_put(struct amdgpu_usermode_queue *queue)
> +{
> +	if (queue)
> +		kref_put(&queue->refcount, amdgpu_userq_kref_destroy);
> +}
>  
>  static int amdgpu_userq_priority_permit(struct drm_file *filp,
>  					int priority)
> @@ -891,6 +905,8 @@ amdgpu_userq_create(struct drm_file *filp, union drm_amdgpu_userq *args)
>  
>  	args->out.queue_id = qid;
>  	atomic_inc(&uq_mgr->userq_count[queue->queue_type]);
> +	/* drop this refcount during queue destroy */
> +	kref_init(&queue->refcount);
>  
>  unlock:
>  	mutex_unlock(&uq_mgr->userq_mutex);
> @@ -985,6 +1001,8 @@ int amdgpu_userq_ioctl(struct drm_device *dev, void *data,
>  		       struct drm_file *filp)
>  {
>  	union drm_amdgpu_userq *args = data;
> +	struct amdgpu_fpriv *fpriv = filp->driver_priv;
> +	struct amdgpu_usermode_queue *queue;
>  	int r;
>  
>  	if (!amdgpu_userq_enabled(dev))
> @@ -1000,11 +1018,14 @@ int amdgpu_userq_ioctl(struct drm_device *dev, void *data,
>  			drm_file_err(filp, "Failed to create usermode queue\n");
>  		break;
>  
> -	case AMDGPU_USERQ_OP_FREE:
> -		r = amdgpu_userq_destroy(filp, args->in.queue_id);
> -		if (r)
> -			drm_file_err(filp, "Failed to destroy usermode queue\n");
> +	case AMDGPU_USERQ_OP_FREE: {
> +		queue = xa_erase(&fpriv->userq_mgr.userq_xa, args->in.queue_id);

You need to have xa_lock around that or otherwise xa_load/kref_get can race.

Apart of that Reviewed-by: Christian König <christian.koenig@amd.com>

Regards,
Christian.

> +		if (!queue)
> +			return -ENOENT;
> +
> +		amdgpu_userq_put(queue);
>  		break;
> +	}
>  
>  	default:
>  		drm_dbg_driver(dev, "Invalid user queue op specified: %d\n", args->in.op);
> @@ -1023,16 +1044,23 @@ amdgpu_userq_restore_all(struct amdgpu_userq_mgr *uq_mgr)
>  
>  	/* Resume all the queues for this process */
>  	xa_for_each(&uq_mgr->userq_xa, queue_id, queue) {
> +		queue = amdgpu_userq_get(uq_mgr, queue_id);
> +		if (!queue)
> +			continue;
> +
>  		if (!amdgpu_userq_buffer_vas_mapped(queue)) {
>  			drm_file_err(uq_mgr->file,
>  				     "trying restore queue without va mapping\n");
>  			queue->state = AMDGPU_USERQ_STATE_INVALID_VA;
> +			amdgpu_userq_put(queue);
>  			continue;
>  		}
>  
>  		r = amdgpu_userq_restore_helper(queue);
>  		if (r)
>  			ret = r;
> +
> +		amdgpu_userq_put(queue);
>  	}
>  
>  	if (ret)
> @@ -1266,9 +1294,13 @@ amdgpu_userq_evict_all(struct amdgpu_userq_mgr *uq_mgr)
>  	amdgpu_userq_detect_and_reset_queues(uq_mgr);
>  	/* Try to unmap all the queues in this process ctx */
>  	xa_for_each(&uq_mgr->userq_xa, queue_id, queue) {
> +		queue = amdgpu_userq_get(uq_mgr, queue_id);
> +		if (!queue)
> +			continue;
>  		r = amdgpu_userq_preempt_helper(queue);
>  		if (r)
>  			ret = r;
> +		amdgpu_userq_put(queue);
>  	}
>  
>  	if (ret)
> @@ -1301,16 +1333,24 @@ amdgpu_userq_wait_for_signal(struct amdgpu_userq_mgr *uq_mgr)
>  	int ret;
>  
>  	xa_for_each(&uq_mgr->userq_xa, queue_id, queue) {
> +		queue = amdgpu_userq_get(uq_mgr, queue_id);
> +		if (!queue)
> +			continue;
> +
>  		struct dma_fence *f = queue->last_fence;
>  
> -		if (!f || dma_fence_is_signaled(f))
> +		if (!f || dma_fence_is_signaled(f)) {
> +			amdgpu_userq_put(queue);
>  			continue;
> +		}
>  		ret = dma_fence_wait_timeout(f, true, msecs_to_jiffies(100));
>  		if (ret <= 0) {
>  			drm_file_err(uq_mgr->file, "Timed out waiting for fence=%llu:%llu\n",
>  				     f->context, f->seqno);
> +			amdgpu_userq_put(queue);
>  			return -ETIMEDOUT;
>  		}
> +		amdgpu_userq_put(queue);
>  	}
>  
>  	return 0;
> @@ -1361,20 +1401,24 @@ int amdgpu_userq_mgr_init(struct amdgpu_userq_mgr *userq_mgr, struct drm_file *f
>  void amdgpu_userq_mgr_fini(struct amdgpu_userq_mgr *userq_mgr)
>  {
>  	struct amdgpu_usermode_queue *queue;
> -	unsigned long queue_id;
> +	unsigned long queue_id = 0;
>  
>  	cancel_delayed_work_sync(&userq_mgr->resume_work);
> +	for (;;) {
> +		xa_lock(&userq_mgr->userq_xa);
> +		queue = xa_find(&userq_mgr->userq_xa, &queue_id, ULONG_MAX,
> +				XA_PRESENT);
> +		if (queue)
> +			__xa_erase(&userq_mgr->userq_xa, queue_id);
> +		xa_unlock(&userq_mgr->userq_xa);
> +
> +		if (!queue)
> +			break;
>  
> -	mutex_lock(&userq_mgr->userq_mutex);
> -	amdgpu_userq_detect_and_reset_queues(userq_mgr);
> -	xa_for_each(&userq_mgr->userq_xa, queue_id, queue) {
> -		amdgpu_userq_wait_for_last_fence(queue);
> -		amdgpu_userq_unmap_helper(queue);
> -		amdgpu_userq_cleanup(queue, queue_id);
> +		amdgpu_userq_put(queue);
>  	}
>  
>  	xa_destroy(&userq_mgr->userq_xa);
> -	mutex_unlock(&userq_mgr->userq_mutex);
>  	mutex_destroy(&userq_mgr->userq_mutex);
>  }
>  
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
> index f4d1d46ae15e..54e1997b3cc0 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq.h
> @@ -74,6 +74,7 @@ struct amdgpu_usermode_queue {
>  	struct dentry		*debugfs_queue;
>  	struct delayed_work hang_detect_work;
>  	struct dma_fence *hang_detect_fence;
> +	struct kref		refcount;
>  
>  	struct list_head	userq_va_list;
>  };
> @@ -114,6 +115,9 @@ struct amdgpu_db_info {
>  	struct amdgpu_userq_obj	*db_obj;
>  };
>  
> +struct amdgpu_usermode_queue *amdgpu_userq_get(struct amdgpu_userq_mgr *uq_mgr, u32 qid);
> +void amdgpu_userq_put(struct amdgpu_usermode_queue *queue);
> +
>  int amdgpu_userq_ioctl(struct drm_device *dev, void *data, struct drm_file *filp);
>  
>  int amdgpu_userq_mgr_init(struct amdgpu_userq_mgr *userq_mgr, struct drm_file *file_priv,
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> index 3c30512a6266..a7ded25346b8 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_userq_fence.c
> @@ -470,7 +470,7 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data,
>  	struct drm_gem_object **gobj_write, **gobj_read;
>  	u32 *syncobj_handles, num_syncobj_handles;
>  	struct amdgpu_userq_fence *userq_fence;
> -	struct amdgpu_usermode_queue *queue;
> +	struct amdgpu_usermode_queue *queue = NULL;
>  	struct drm_syncobj **syncobj = NULL;
>  	struct dma_fence *fence;
>  	struct drm_exec exec;
> @@ -521,7 +521,7 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data,
>  		goto put_gobj_read;
>  
>  	/* Retrieve the user queue */
> -	queue = xa_load(&userq_mgr->userq_xa, args->queue_id);
> +	queue = amdgpu_userq_get(userq_mgr, args->queue_id);
>  	if (!queue) {
>  		r = -ENOENT;
>  		goto put_gobj_write;
> @@ -612,6 +612,9 @@ int amdgpu_userq_signal_ioctl(struct drm_device *dev, void *data,
>  free_syncobj_handles:
>  	kfree(syncobj_handles);
>  
> +	if (queue)
> +		amdgpu_userq_put(queue);
> +
>  	return r;
>  }
>  
> @@ -626,7 +629,7 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>  	struct amdgpu_userq_mgr *userq_mgr = &fpriv->userq_mgr;
>  	struct drm_gem_object **gobj_write, **gobj_read;
>  	u32 *timeline_points, *timeline_handles;
> -	struct amdgpu_usermode_queue *waitq;
> +	struct amdgpu_usermode_queue *waitq = NULL;
>  	u32 *syncobj_handles, num_syncobj;
>  	struct dma_fence **fences = NULL;
>  	u16 num_points, num_fences = 0;
> @@ -863,7 +866,7 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>  		 */
>  		num_fences = dma_fence_dedup_array(fences, num_fences);
>  
> -		waitq = xa_load(&userq_mgr->userq_xa, wait_info->waitq_id);
> +		waitq = amdgpu_userq_get(userq_mgr, wait_info->waitq_id);
>  		if (!waitq) {
>  			r = -EINVAL;
>  			goto free_fences;
> @@ -947,5 +950,8 @@ int amdgpu_userq_wait_ioctl(struct drm_device *dev, void *data,
>  free_syncobj_handles:
>  	kfree(syncobj_handles);
>  
> +	if (waitq)
> +		amdgpu_userq_put(waitq);
> +
>  	return r;
>  }