Message-ID: <2024122737-CVE-2024-56644-83c2@gregkh>
Date: Fri, 27 Dec 2024 16:02:45 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2024-56644: net/ipv6: release expired exception dst cached in socket

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

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

net/ipv6: release expired exception dst cached in socket

Dst objects get leaked in ip6_negative_advice() when this function is
executed for an expired IPv6 route located in the exception table. There
are several conditions that must be fulfilled for the leak to occur:
* an ICMPv6 packet indicating a change of the MTU for the path is received,
  resulting in an exception dst being created
* a TCP connection that uses the exception dst for routing packets must
  start timing out so that TCP begins retransmissions
* after the exception dst expires, the FIB6 garbage collector must not run
  before TCP executes ip6_negative_advice() for the expired exception dst

When TCP executes ip6_negative_advice() for an exception dst that has
expired and if no other socket holds a reference to the exception dst, the
refcount of the exception dst is 2, which corresponds to the increment
made by dst_init() and the increment made by the TCP socket for which the
connection is timing out. The refcount made by the socket is never
released. The refcount of the dst is decremented in sk_dst_reset() but
that decrement is counteracted by a dst_hold() intentionally placed just
before the sk_dst_reset() in ip6_negative_advice(). After
ip6_negative_advice() has finished, there is no other object tied to the
dst. The socket lost its reference stored in sk_dst_cache and the dst is
no longer in the exception table. The exception dst becomes a leaked
object.

As a result of this dst leak, an unbalanced refcount is reported for the
loopback device of a net namespace being destroyed under kernels that do
not contain e5f80fcf869a ("ipv6: give an IPv6 dev to blackhole_netdev"):
unregister_netdevice: waiting for lo to become free. Usage count = 2

Fix the dst leak by removing the dst_hold() in ip6_negative_advice(). The
patch that introduced the dst_hold() in ip6_negative_advice() was
92f1655aa2b22 ("net: fix __dst_negative_advice() race"). But 92f1655aa2b22
merely refactored the code with regards to the dst refcount so the issue
was present even before 92f1655aa2b22. The bug was introduced in
54c1a859efd9f ("ipv6: Don't drop cache route entry unless timer actually
expired.") where the expired cached route is deleted and the sk_dst_cache
member of the socket is set to NULL by calling dst_negative_advice() but
the refcount belonging to the socket is left unbalanced.

The IPv4 version - ipv4_negative_advice() - is not affected by this bug.
When the TCP connection times out ipv4_negative_advice() merely resets the
sk_dst_cache of the socket while decrementing the refcount of the
exception dst.

The Linux kernel CVE team has assigned CVE-2024-56644 to this issue.


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

	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 5.4.287 with commit a95808252e8acc0123bacd2dff8b9af10bc145b7
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 5.10.231 with commit b90d061345bb8cd51fece561a800bae1c95448a6
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 5.15.174 with commit 0b8903e6c881f72c6849d4952de742c656eb5ab9
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 6.1.120 with commit 535add1e9f274502209cb997801208bbe1ae6c6f
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 6.6.66 with commit f43d12fd0fa8ee5b9caf8a3927e10d06431764d2
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 6.12.5 with commit 8b591bd522b71c42a82898290e35d32b482047e4
	Issue introduced in 2.6.34 with commit 54c1a859efd9fd6cda05bc700315ba2519c14eba and fixed in 6.13-rc2 with commit 3301ab7d5aeb0fe270f73a3d4810c9d1b6a9f045
	Issue introduced in 2.6.33.2 with commit 9c93e9c757c7d3d96027a06b9b4c4e37ca87ded7

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-2024-56644
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:
	net/ipv6/route.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/a95808252e8acc0123bacd2dff8b9af10bc145b7
	https://git.kernel.org/stable/c/b90d061345bb8cd51fece561a800bae1c95448a6
	https://git.kernel.org/stable/c/0b8903e6c881f72c6849d4952de742c656eb5ab9
	https://git.kernel.org/stable/c/535add1e9f274502209cb997801208bbe1ae6c6f
	https://git.kernel.org/stable/c/f43d12fd0fa8ee5b9caf8a3927e10d06431764d2
	https://git.kernel.org/stable/c/8b591bd522b71c42a82898290e35d32b482047e4
	https://git.kernel.org/stable/c/3301ab7d5aeb0fe270f73a3d4810c9d1b6a9f045

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