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

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

[Xiaoguang Wang]

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 |

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

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

[Stefano Garzarella]

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.

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
> 

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

[Xiaoguang Wang]

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
>>

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

[Stefano Garzarella]

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?

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
> > > 
> 

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

[Xiaoguang Wang]

> 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
>>>>
>>