Re: [PATCH 2/3] io_uring: fix io_rsrc_ref_quiesce races

[Pavel Begunkov <asml.silence@gmail.com>]

On 20/02/2021 14:07, Pavel Begunkov wrote:
> On 20/02/2021 06:31, Hao Xu wrote:
>> 在 2021/2/20 上午4:45, Pavel Begunkov 写道:
>>> There are different types of races in io_rsrc_ref_quiesce()  between
>>> ->release() of percpu_refs and reinit_completion(), fix them by always
>>> resurrecting between iterations. BTW, clean the function up, because
>>> DRY.
>>>
>>> Fixes: 0ce4a72632317 ("io_uring: don't hold uring_lock when calling io_run_task_work*")
>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>> ---
>>> -        /*
>>> -         * There is small possibility that data->done is already completed
>>> -         * So reinit it here
>>> -         */
>>> +        percpu_ref_resurrect(&data->refs);
>> I've thought about this, if we resurrect data->refs, then we can't
>> avoid parallel files unregister by percpu_refs_is_dying.
> 
> Right, totally forgot about it, but otherwise we race with data->done.
> Didn't test yet, but a diff below should do the trick.
> 
>>> +        if (ret < 0)
>>> +            return ret;
>>> +        backup_node = alloc_fixed_rsrc_ref_node(ctx);
>>> +        if (!backup_node)
>>> +            return -ENOMEM;
>> Should we resurrect data->refs and reinit completion before
>> signal or error return?
> 
> Not sure what exactly you mean, we should not release uring_lock with
> inconsistent state, so it's done right before unlock. And we should not
> do it before wait_for_completion_interruptible() before it would take a
> reference.
> 
>>> +        init_fixed_file_ref_node(ctx, backup_node);
>>> +    } while (1);
>>>         destroy_fixed_rsrc_ref_node(backup_node);
>>>       return 0;

upd

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ce5fccf00367..12796de8ad10 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -236,6 +236,7 @@ struct fixed_rsrc_data {
 	struct fixed_rsrc_ref_node	*node;
 	struct percpu_ref		refs;
 	struct completion		done;
+	bool				quiesce;
 };
 
 struct io_buffer {
@@ -7335,10 +7336,12 @@ static int io_rsrc_ref_quiesce(struct fixed_rsrc_data *data,
 	struct fixed_rsrc_ref_node *backup_node;
 	int ret;
 
+	data->quiesce = true;
 	do {
 		backup_node = alloc_fixed_rsrc_ref_node(ctx);
+		ret = -ENOMEM;
 		if (!backup_node)
-			return -ENOMEM;
+			break;
 		backup_node->rsrc_data = data;
 		backup_node->rsrc_put = rsrc_put;
 
@@ -7353,16 +7356,17 @@ static int io_rsrc_ref_quiesce(struct fixed_rsrc_data *data,
 		percpu_ref_resurrect(&data->refs);
 		synchronize_rcu();
 		io_sqe_rsrc_set_node(ctx, data, backup_node);
+		backup_node = NULL;
 		reinit_completion(&data->done);
 		mutex_unlock(&ctx->uring_lock);
 		ret = io_run_task_work_sig();
 		mutex_lock(&ctx->uring_lock);
-		if (ret < 0)
-			return ret;
-	} while (1);
+	} while (ret >= 0);
 
-	destroy_fixed_rsrc_ref_node(backup_node);
-	return 0;
+	data->quiesce = false;
+	if (backup_node)
+		destroy_fixed_rsrc_ref_node(backup_node);
+	return ret;
 }
 
 static struct fixed_rsrc_data *alloc_fixed_rsrc_data(struct io_ring_ctx *ctx)
@@ -7401,7 +7405,7 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 	 * Since we possibly drop uring lock later in this function to
 	 * run task work.
 	 */
-	if (!data || percpu_ref_is_dying(&data->refs))
+	if (!data || data->quiesce)
 		return -ENXIO;
 	ret = io_rsrc_ref_quiesce(data, ctx, io_ring_file_put);
 	if (ret)


-- 
Pavel Begunkov