Re: [Xen-devel] [PATCH 1/3] xen/pv-on-hvm kexec: prevent crash in xenwatch_thread() when stale watch events arrive

[Ian Campbell <Ian.Campbell@citrix.com>]

On Tue, 2011-08-16 at 14:16 +0100, Olaf Hering wrote:
> During repeated kexec boots xenwatch_thread() can crash because
> xenbus_watch->callback is cleared by xenbus_watch_path() if a node/token
> combo for a new watch happens to match an already registered watch from
> an old kernel.  In this case xs_watch returns -EEXISTS, then
> register_xenbus_watch() does not remove the to-be-registered watch from
> the list of active watches but returns the -EEXISTS to the caller
> anyway.

Isn't this behaviour the root cause of the issue (which should be fixed)
rather than papering over it during watch processing. IOW should't
register_xenbus_watch cleanup after itself if xs_watch fails.

> 
> Because the watch is still active in xenstored it will cause an event
> which will arrive in the new kernel. process_msg() will find the
> encapsulated struct xenbus_watch in its list of registered watches and
> puts the "empty" watch handle in the queue for xenwatch_thread().
> xenwatch_thread() then calls ->callback which was cleared earlier by
> xenbus_watch_path().
> 
> To prevent that crash in a guest running on an old xen toolstack, add a
> check wether xenbus_watch->callback is active.
> 
> Signed-off-by: Olaf Hering <olaf@aepfle.de>
> ---
>  drivers/xen/xenbus/xenbus_xs.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c
> index 5534690..64248b2 100644
> --- a/drivers/xen/xenbus/xenbus_xs.c
> +++ b/drivers/xen/xenbus/xenbus_xs.c
> @@ -828,7 +828,7 @@ static int process_msg(void)
>  		spin_lock(&watches_lock);
>  		msg->u.watch.handle = find_watch(
>  			msg->u.watch.vec[XS_WATCH_TOKEN]);
> -		if (msg->u.watch.handle != NULL) {
> +		if (msg->u.watch.handle && msg->u.watch.handle->callback) {
>  			spin_lock(&watch_events_lock);
>  			list_add_tail(&msg->list, &watch_events);
>  			wake_up(&watch_events_waitq);