linux-cve-announce - CVE-2025-37949: xenbus: Use kref to track req lifetime

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Message-ID: <2025052000-CVE-2025-37949-c272@gregkh>
Date: Tue, 20 May 2025 18:02:03 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-37949: xenbus: Use kref to track req lifetime

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

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

The Linux kernel CVE team has assigned CVE-2025-37949 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 5.15.183 with commit 4d260a5558df4650eb87bc41b2c9ac2d6b2ba447
	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 6.1.139 with commit 8b02f85e84dc6f7c150cef40ddb69af5a25659e5
	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 6.6.91 with commit cbfaf46b88a4c01b64c4186cdccd766c19ae644c
	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 6.12.29 with commit 8e9c8a0393b5f85f1820c565ab8105660f4e8f92
	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 6.14.7 with commit 2466b0f66795c3c426cacc8998499f38031dbb59
	Issue introduced in 4.11 with commit fd8aa9095a95c02dcc35540a263267c29b8fda9d and fixed in 6.15-rc6 with commit 1f0304dfd9d217c2f8b04a9ef4b3258a66eedd27

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-37949
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/xen/xenbus/xenbus.h
	drivers/xen/xenbus/xenbus_comms.c
	drivers/xen/xenbus/xenbus_dev_frontend.c
	drivers/xen/xenbus/xenbus_xs.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4d260a5558df4650eb87bc41b2c9ac2d6b2ba447
	https://git.kernel.org/stable/c/8b02f85e84dc6f7c150cef40ddb69af5a25659e5
	https://git.kernel.org/stable/c/cbfaf46b88a4c01b64c4186cdccd766c19ae644c
	https://git.kernel.org/stable/c/8e9c8a0393b5f85f1820c565ab8105660f4e8f92
	https://git.kernel.org/stable/c/2466b0f66795c3c426cacc8998499f38031dbb59
	https://git.kernel.org/stable/c/1f0304dfd9d217c2f8b04a9ef4b3258a66eedd27