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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe