Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

[Jens Axboe <axboe@kernel.dk>]

On 10/18/19 9:00 AM, Jens Axboe wrote:
> On 10/18/19 8:52 AM, Jann Horn wrote:
>> On Fri, Oct 18, 2019 at 4:43 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>
>>> On 10/18/19 8:40 AM, Jann Horn wrote:
>>>> On Fri, Oct 18, 2019 at 4:37 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>
>>>>> On 10/18/19 8:34 AM, Jann Horn wrote:
>>>>>> On Fri, Oct 18, 2019 at 4:01 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>> On 10/17/19 8:41 PM, Jann Horn wrote:
>>>>>>>> On Fri, Oct 18, 2019 at 4:01 AM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>>> This is in preparation for adding opcodes that need to modify files
>>>>>>>>> in a process file table, either adding new ones or closing old ones.
>>>>>> [...]
>>>>>>> Updated patch1:
>>>>>>>
>>>>>>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=df6caac708dae8ee9a74c9016e479b02ad78d436
>>>>>>
>>>>>> I don't understand what you're doing with old_files in there. In the
>>>>>> "s->files && !old_files" branch, "current->files = s->files" happens
>>>>>> without holding task_lock(), but current->files and s->files are also
>>>>>> the same already at that point anyway. And what's the intent behind
>>>>>> assigning stuff to old_files inside the loop? Isn't that going to
>>>>>> cause the workqueue to keep a modified current->files beyond the
>>>>>> runtime of the work?
>>>>>
>>>>> I simply forgot to remove the old block, it should only have this one:
>>>>>
>>>>> if (s->files && s->files != cur_files) {
>>>>>            task_lock(current);
>>>>>            current->files = s->files;
>>>>>            task_unlock(current);
>>>>>            if (cur_files)
>>>>>                    put_files_struct(cur_files);
>>>>>            cur_files = s->files;
>>>>> }
>>>>
>>>> Don't you still need a put_files_struct() in the case where "s->files
>>>> == cur_files"?
>>>
>>> I want to hold on to the files for as long as I can, to avoid unnecessary
>>> shuffling of it. But I take it your worry here is that we'll be calling
>>> something that manipulates ->files? Nothing should do that, unless
>>> s->files is set. We didn't hide the workqueue ->files[] before this
>>> change either.
>>
>> No, my worry is that the refcount of the files_struct is left too
>> high. From what I can tell, the "do" loop in io_sq_wq_submit_work()
>> iterates over multiple instances of struct sqe_submit. If there are
>> two sqe_submit instances with the same ->files (each holding a
>> reference from the get_files_struct() in __io_queue_sqe()), then:
>>
>> When processing the first sqe_submit instance, current->files and
>> cur_files are set to $user_files.
>> When processing the second sqe_submit instance, nothing happens
>> (s->files == cur_files).
>> After the loop, at the end of the function, put_files_struct() is
>> called once on $user_files.
>>
>> So get_files_struct() has been called twice, but put_files_struct()
>> has only been called once. That leaves the refcount too high, and by
>> repeating this, an attacker can make the refcount wrap around and then
>> cause a use-after-free.
> 
> Ah now I see what you are getting at, yes that's clearly a bug! I wonder
> how we best safely can batch the drops. We can track the number of times
> we've used the same files, and do atomic_sub_and_test() in a
> put_files_struct_many() type addition. But that would leave us open to
> the issue you describe, where someone could maliciously overflow the
> files ref count.
> 
> Probably not worth over-optimizing, as long as we can avoid the
> current->files task lock/unlock and shuffle.
> 
> I'll update the patch.

Alright, this incremental on top should do it. And full updated patch
here:

http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=40449c5a3d3b16796fa13e9469c69d62986e961c

Let me know what you think.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 68eda36bcc9c..2fed0badad38 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2289,13 +2289,13 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 				set_fs(USER_DS);
 			}
 		}
-		if (s->files && s->files != cur_files) {
+		if (cur_files)
+			put_files_struct(cur_files);
+		cur_files = s->files;
+		if (cur_files && cur_files != current->files) {
 			task_lock(current);
-			current->files = s->files;
+			current->files = cur_files;
 			task_unlock(current);
-			if (cur_files)
-				put_files_struct(cur_files);
-			cur_files = s->files;
 		}
 
 		if (!ret) {
@@ -2393,8 +2393,9 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 		task_lock(current);
 		current->files = old_files;
 		task_unlock(current);
-		put_files_struct(cur_files);
 	}
+	if (cur_files)
+		put_files_struct(cur_files);
 }
 
 /*

-- 
Jens Axboe