Re: [PATCH] nbd: fix potential NULL pointer fault in connect and disconnect process

[sunke (E)]

Hi Mike

Your idea looks good.

Thanks,
Sun Ke

å 2020/2/10 11:16, Mike Christie åé:
> On 01/19/2020 01:10 AM, sunke (E) wrote:
> > Thanks for your detailed suggestions.
> >
> > å 2020/1/18 1:32, Mike Christie åé:
> > > On 01/17/2020 05:50 AM, Sun Ke wrote:
> > > > Connect and disconnect a nbd device repeatedly, will cause
> > > > NULL pointer fault.
> > > >
> > > > It will appear by the steps:
> > > > 1. Connect the nbd device and disconnect it, but now nbd device
> > > >      is not disconnected totally.
> > > > 2. Connect the same nbd device again immediately, it will fail
> > > >      in nbd_start_device with a EBUSY return value.
> > > > 3. Wait a second to make sure the last config_refs is reduced
> > > >      and run nbd_config_put to disconnect the nbd device totally.
> > > > 4. Start another process to open the nbd_device, config_refs
> > > >      will increase and at the same time disconnect it.
> > >
> > > Just to make sure I understood this, for step 4 the process is doing:
> > >
> > > open(/dev/nbdX);
> > > ioctl(NBD_DISCONNECT, /dev/nbdX) or nbd_genl_disconnect(for /dev/nbdX)
> > >
> > > ?
> > do nbd_genl_disconnect(for /dev/nbdX)ï
> > I tested it. Connect /dev/nbdX
> > through ioctl interface by nbd-client -L -N export localhost /dev/nbdX and
> > through netlink interface by nbd-client localhost XXXX /dev/nbdX,
> > disconnect /dev/nbdX by nbd-client -d /dev/nbdX.
> > Both call nbd_genl_disconnect(for /dev/nbdX) and both contain the same
> > null pointer dereference.
> > > There is no successful NBD_DO_IT / nbd_genl_connect between the open and
> > > disconnect calls at step #4, because it would normally be done at #2 and
> > > that failed. nbd_disconnect_and_put could then reference a null
> > > recv_workq. If we are also racing with a close() then that could free
> > > the device/config from under nbd_disconnect_and_put.
> > Yes, nbd_disconnect_and_put could then reference a null recv_workq.
>
> Hey Sunke
>
> How about the attached patch. I am still testing it. The basic idea is
> that we need to do a flush whenever we have done a sock_shutdown and are
> in the disconnect/connect/clear sock path, so it just adds the flush in
> that function. We then do not need to keep adding these flushes everywhere.