Bug 1905635 (CVE-2020-29484)

When a Xenstore watch fires, the xenstore client which registered the
watch will receive a Xenstore message containing the path of the
modified Xenstore entry which triggered the watch, and the tag which
was specified when registering the watch.

Any communication with xenstored is done via Xenstore messages,
consisting of a message header and the payload. The payload length is
limited to 4096 bytes. Any request to xenstored resulting in a
response with a payload longer than 4096 bytes will result in an
error.

When registering a watch the payload length limit applies to the
combined length of the watched path and the specified tag. As watches
for a specific path are also triggered for all nodes below that path,
the payload of a watch event message can be longer than the payload
needed to register the watch.

A malicious guest which registers a watch using a very large tag (ie
with a registration operation payload length close to the 4096 byte
limit) can cause the generation of watch events with a payload length
larger than 4096 bytes, by writing to Xenstore entries below the
watched path.

This will result in an error condition in xenstored.  This error can
result in a NULL pointer dereference leading to a crash of xenstored.