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

[Stefano Garzarella <sgarzare@redhat.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?

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
> > > 
>