Re: [PATCH] io_uring: fix possible use after free to sqd

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

> On Thu, Oct 15, 2020 at 06:46:34PM +0800, Xiaoguang Wang wrote:
>> hi閿涳拷
>>
>>> On Thu, Oct 15, 2020 at 05:13:35PM +0800, Xiaoguang Wang wrote:
>>>> Reading codes finds a possible use after free issue to sqd:
>>>>             thread1              |       thread2
>>>> ==> io_attach_sq_data()        |
>>>> ===> sqd = ctx_attach->sq_data;|
>>>>                                  | ==> io_put_sq_data()
>>>>                                  | ===> refcount_dec_and_test(&sqd->refs)
>>>>                                  |     If sqd->refs is zero, will free sqd.
>>>>                                  |
>>>> ===> refcount_inc(&sqd->refs); |
>>>>                                  |
>>>>                                  | ====> kfree(sqd);
>>>> ===> now use after free to sqd |
>>>>
>>>
>>> IIUC the io_attach_sq_data() is called only during the setup of an
>>> io_uring context, before that the fd is returned to the user space.
>> Sorry I didn't make it clear in commit message.
>> In io_attach_sq_data(), we'll try to attach to a previous io_uring instance
>> indicated by p->wq_fd, this p->wq_fd could be closed independently.
> 
> Thanks to clarify! Got it.
> 
> Also in this case, IIUC io_put_sq_data() is called only when
> io_uring_release() is invoked on the previous io_uring instance.
> 
> Since we are taking a reference with fdget at the begin of io_attach_sq_data()
> and we release this reference only at the end of io_attach_sq_data(),
> can io_put_sq_data() runs in another thread while we have the reference?
Oh, right, thanks. Jens, you can ignore this patch, thanks.

Regards,
Xiaoguang Wang

> 
> Thanks,
> Stefano
> 
>>
>> Regards,
>> Xiaoguang Wang
>>>
>>> So, I'm not sure a second thread can call io_put_sq_data() while the
>>> first thread is in io_attach_sq_data().
>>>
>>> Can you check if this situation can really happen?
>>>
>>> Thanks,
>>> Stefano
>>>
>>>> Use refcount_inc_not_zero() to fix this issue.
>>>>
>>>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>>>> ---
>>>>    fs/io_uring.c | 6 +++++-
>>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 33b5cf18bb51..48e230feb704 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -6868,7 +6868,11 @@ static struct io_sq_data *io_attach_sq_data(struct io_uring_params *p)
>>>>    		return ERR_PTR(-EINVAL);
>>>>    	}
>>>> -	refcount_inc(&sqd->refs);
>>>> +	if (!refcount_inc_not_zero(&sqd->refs)) {
>>>> +		fdput(f);
>>>> +		return ERR_PTR(-EINVAL);
>>>> +	}
>>>> +
>>>>    	fdput(f);
>>>>    	return sqd;
>>>>    }
>>>> -- 
>>>> 2.17.2
>>>>
>>