Ubuntu: (Multiple Advisories) (CVE-2025-37949): Linux kernel vulnerabilities

In the Linux kernel, the following vulnerability has been resolved:

xenbus: Use kref to track req lifetime

Marek reported seeing a NULL pointer fault in the xenbus_thread
callstack:
BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: e030:__wake_up_common+0x4c/0x180
Call Trace:
<TASK>
__wake_up_common_lock+0x82/0xd0
process_msg+0x18e/0x2f0
xenbus_thread+0x165/0x1c0

process_msg+0x18e is req->cb(req). req->cb is set to xs_wake_up(), a
thin wrapper around wake_up(), or xenbus_dev_queue_reply(). It seems
like it was xs_wake_up() in this case.

It seems like req may have woken up the xs_wait_for_reply(), which
kfree()ed the req. When xenbus_thread resumes, it faults on the zero-ed
data.

Linux Device Drivers 2nd edition states:
"Normally, a wake_up call can cause an immediate reschedule to happen,
meaning that other processes might run before wake_up returns."
... which would match the behaviour observed.

Change to keeping two krefs on each request. One for the caller, and
one for xenbus_thread. Each will kref_put() when finished, and the last
will free it.

This use of kref matches the description in
Documentation/core-api/kref.rst