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

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

hi，

> 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.

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
>>