[Xen-devel] [PATCH 2 of 3] linux-2.6.18: xen/pv-on-hvm kexec: prevent crash in xenwatch_thread() when stale watch events arrive

[Olaf Hering <olaf@xxxxxxxxx>]

# HG changeset patch
# User Olaf Hering <olaf@xxxxxxxxx>
# Date 1314289811 -7200
# Node ID 94943cf143035aa7adbec2c39c45c16a5174e3c5
# Parent  86e85596d64b49a9a9c07e8817c33d046b078fd4
linux-2.6.18: xen/pv-on-hvm kexec: prevent crash in xenwatch_thread() when 
stale watch events arrive

commit c4c303c7c5679b4b368e12f41124aee29c325b76

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.

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 remove
the special -EEXIST handling.

v2:
 - remove the EEXIST handing in register_xenbus_watch() instead of
   checking for ->callback in process_msg()

Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>

diff -r 86e85596d64b -r 94943cf14303 drivers/xen/xenbus/xenbus_xs.c
--- a/drivers/xen/xenbus/xenbus_xs.c
+++ b/drivers/xen/xenbus/xenbus_xs.c
@@ -656,8 +656,7 @@ int register_xenbus_watch(struct xenbus_
 
        err = xs_watch(watch->node, token);
 
-       /* Ignore errors due to multiple registration. */
-       if ((err != 0) && (err != -EEXIST)) {
+       if (err) {
                spin_lock(&watches_lock);
                list_del(&watch->list);
                spin_unlock(&watches_lock);

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