Re: [PATCH] nbd: pass nbd_sock to nbd_read_reply() instead of index

[Ming Lei <ming.lei@redhat.com>]

On Thu, Sep 28, 2023 at 02:03:28PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2023/09/28 12:05, Ming Lei 写道:
> > On Mon, Sep 11, 2023 at 10:33:08AM +0800, linan666@huaweicloud.com wrote:
> > > From: Li Nan <linan122@huawei.com>
> > > 
> > > If a socket is processing ioctl 'NBD_SET_SOCK', config->socks might be
> > > krealloc in nbd_add_socket(), and a garbage request is received now, a UAF
> > > may occurs.
> > > 
> > >    T1
> > >    nbd_ioctl
> > >     __nbd_ioctl
> > >      nbd_add_socket
> > >       blk_mq_freeze_queue
> > > 				T2
> > >    				recv_work
> > >    				 nbd_read_reply
> > >    				  sock_xmit
> > >       krealloc config->socks
> > > 				   def config->socks
> > > 
> > > Pass nbd_sock to nbd_read_reply(). And introduce a new function
> > > sock_xmit_recv(), which differs from sock_xmit only in the way it get
> > > socket.
> > > 
> > 
> > I am wondering why not grab queue usage counter before calling nbd_read_reply()
> > for avoiding such issue, something like the following change:
> > 
> > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > index df1cd0f718b8..09215b605b12 100644
> > --- a/drivers/block/nbd.c
> > +++ b/drivers/block/nbd.c
> > @@ -837,9 +837,6 @@ static void recv_work(struct work_struct *work)
> >   	while (1) {
> >   		struct nbd_reply reply;
> > -		if (nbd_read_reply(nbd, args->index, &reply))
> > -			break;
> > -
> >   		/*
> >   		 * Grab .q_usage_counter so request pool won't go away, then no
> >   		 * request use-after-free is possible during nbd_handle_reply().
> > @@ -852,6 +849,9 @@ static void recv_work(struct work_struct *work)
> >   			break;
> >   		}
> 
> This break how nbd works, if there is no reply yet, recv_work() will
> wait for reply in:
> 
> nbd_read_reply
>  sock_xmit
>   sock_recvmsg
> 
> After this change, recv_work() will just return if there is no io.

OK, got it, thanks for the input.

But I feel it isn't necessary & fragile to store one extra reference of nsock in
`recv_thread_args`.

Just run a quick look, the only potential UAF on config->socks should be recv_work(),
so you can retrieve the `nsock` reference at the entry of recv_work(),
and just pass it(local variable) to nbd_read_reply() and nbd_handle_reply()
since `nsock` won't be freed.


Thanks,
Ming