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

[Ming Lei]

On Thu, Sep 28, 2023 at 05:06:40PM +0800, Yu Kuai wrote:
> Hi,
> 
> 在 2023/09/28 16:57, Ming Lei 写道:
> > On Thu, Sep 28, 2023 at 04:55:03PM +0800, Yu Kuai wrote:
> > > Hi,
> > > 
> > > 在 2023/09/28 15:40, Ming Lei 写道:
> > > > 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@xxxxxxxxxxxxxxx wrote:
> > > > > > > From: Li Nan <linan122@xxxxxxxxxx>
> > > > > > > 
> > > > > > > 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(),
> > > 
> > > I don't understand what you mean retrieve the 'nsock', is following what
> > > you expected?
> > > 
> > > blk_queue_enter() -> prevent concurrent with nbd_add_socket
> > > nsock = config->socks[args->index]
> > > blk_queue_exit()
> > 
> > Yeah, turns out you do understand, :-)
> 
> Ok, I was not sure about this blk_queue_enter(). By the way, this

blk_queue_enter() isn't exported, but you can grab ->config_lock
for getting the `nsock`.

> remind me of what you did to fix uaf of access queue->mq_hctx[] by
> convert the array to xarray.
> 
> 
> Maybe it's better to covert config->socks[] to xarray to fix this uaf as
> well?

->socks[idx] is needed in nbd fast path, so xarray may not be one good idea
since xarray_load() introduces extra load, especially ->socks[] uaf
should exist in recv_work() very likely. For other cases, the active
block request holds queue usage counter.


Thanks,
Ming