Re: [Xen-devel] [PATCH] xsplice: Don't perform multiple operations on same payload once work is scheduled.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Apr 28, 2016 at 09:52:14PM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Apr 29, 2016 at 12:23:38AM +0100, Andrew Cooper wrote:
> > On 28/04/2016 19:16, Konrad Rzeszutek Wilk wrote:
> > > diff --git a/xen/common/xsplice.c b/xen/common/xsplice.c
> > > index 1b67d39..48088ce 100644
> > > --- a/xen/common/xsplice.c
> > > +++ b/xen/common/xsplice.c
> > > @@ -1099,6 +1099,13 @@ static void xsplice_do_action(void)
> > >      data->rc = rc;
> > >  }
> > >  
> > > +static bool_t is_work_scheduled(struct payload *data)
> > 
> > const struct payload *data
> 
> Yes!
> > 
> > Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
> And of course 5 hours later I realized there is a more straightforward
> for this. It follows the same idea but it piggyback on data->rc
> being set by 'schedule_work' to -EAGAIN once work is scheduled:

Err, 'schedule_work' does not set data->rc to -EAGAIN. It happens
within xsplice_do_action:

        data->rc = -EAGAIN;                                                 
        rc = schedule_work(data, action->cmd, action->timeout);  

(for either replace, revert, or apply).

> 
> It could even be rolled in "xsplice: Implement support for
> applying/reverting/replacing patches."
> 
> 
> >From 83053e3f984b67dfae74cb64e75b8871b9d236ca Mon Sep 17 00:00:00 2001
> From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Date: Thu, 28 Apr 2016 21:22:49 -0400
> Subject: [PATCH] xsplice: Check data->rc for -EAGAIN to guard against races.
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> 
> Currently it is possible to:
> 
> 1)  xc_xsplice_apply()
>      \-> xsplice_action
>         spin_lock(payload_lock)
>              \- schedule_work()
>                  data->rc=-EAGAIN
>         spin_unlock(payload_lock);
> 
> 2)  xc_xsplice_unload()
>      \-> xsplice_action
>         spin_lock(payload_lock)
>              free_payload(data);
>         spin_unlock(payload_lock);
> 
> .. all CPUs are quiesced.
> 
> 3) check_for_xsplice_work()
>      \-> apply_payload
>         \-> arch_xsplice_apply_jmp
>             BOOM
>          data->rc =0
> 
> The reason is that state is in 'CHECKED' which changes to 'APPLIED'
> once check_for_xsplice_work finishes (and it updates data->rc to zero).
> 
> But we have a race between 1) -> 3) where one can manipulate the payload
> (as the state is in 'CHECKED' from which you can apply/revert and unload).
> 
> This patch adds a simple check on data->rc to see if it is in -EAGAIN
> which means that schedule_work has been called for this payload.
> 
> If the payload aborts in check_for_xsplice_work (timed out, etc),
> the data->rc will be -EBUSY -so one can still unload the payload or
> retry the operation.
> 
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Reported-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> ---
> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> CC: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Tim Deegan <tim@xxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ---
> ---
>  xen/common/xsplice.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/xen/common/xsplice.c b/xen/common/xsplice.c
> index 1b67d39..0bc7e0f 100644
> --- a/xen/common/xsplice.c
> +++ b/xen/common/xsplice.c
> @@ -1363,6 +1363,9 @@ static int xsplice_action(xen_sysctl_xsplice_action_t 
> *action)
>          return PTR_ERR(data);
>      }
>  
> +    if ( data->rc == -EAGAIN ) /* schedule_work has been called. */
> +        goto out;
> +
>      switch ( action->cmd )
>      {
>      case XSPLICE_ACTION_UNLOAD:
> @@ -1423,6 +1426,7 @@ static int xsplice_action(xen_sysctl_xsplice_action_t 
> *action)
>          break;
>      }
>  
> + out:
>      spin_unlock(&payload_lock);
>  
>      return rc;
> -- 
> 2.4.0
> 

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