CVE-2026-74735: l2tp: fix tunnel and session refcount leak on seq_file release

CVE-2026-74735: l2tp: fix tunnel and session refcount leak on seq_file release

[Greg Kroah-Hartman]

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

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

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

l2tp: fix tunnel and session refcount leak on seq_file release

In pppol2tp_proc_open() and l2tp_dfs_seq_open(), iteration state
(pd->tunnel and pd->session) is kept in seq_file private data to allow
iteration across multiple read() system calls.

However, if userspace closes /proc/net/pppol2tp or /sys/kernel/debug/l2tp/tunnels
before reading to end-of-file (EOF), any tunnel or session reference stored in
pd->tunnel / pd->session is left un-dropped when seq_file private data is freed.

Fix this by dropping any remaining pd->tunnel and pd->session references in
pppol2tp_proc_release() and l2tp_dfs_seq_release() when closing the file.

The Linux kernel CVE team has assigned CVE-2026-74735 to this issue.


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

	Issue introduced in 4.17 with commit 0e0c3fee3a59a387aeecc4fca6f3a2e9615a5443 and fixed in 7.1.10 with commit ebe2774e956482dd3c70b8991f6f7654356339e6
	Issue introduced in 4.17 with commit 0e0c3fee3a59a387aeecc4fca6f3a2e9615a5443 and fixed in 7.2 with commit 9006c116dd111d457bf5d074990210f70a4ad2c8
	Issue introduced in 3.16.60 with commit f6104e9ae04462523c372928b1173766a771ef95
	Issue introduced in 4.16.6 with commit 7f8e401f9e612df2a95c1ef76217c00181389bb5
	Issue introduced in 3.16.60 with commit c712ed63e9b33d9a282fbd92ce25c64918c3129f
	Issue introduced in 4.16.6 with commit 9a8863e8a8def9d88583cd9c60db38a008e6b8cf

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-2026-74735
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/l2tp/l2tp_debugfs.c
	net/l2tp/l2tp_ppp.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/ebe2774e956482dd3c70b8991f6f7654356339e6
	https://git.kernel.org/stable/c/9006c116dd111d457bf5d074990210f70a4ad2c8