CVE-2025-21792

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

ax25: Fix refcount leak caused by setting SO_BINDTODEVICE sockopt

If an AX25 device is bound to a socket by setting the SO_BINDTODEVICE socket option, a refcount leak will occur in ax25_release().

Commit 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()") added decrement of device refcounts in ax25_release(). In order for that to work correctly the refcounts must already be incremented when the device is bound to the socket. An AX25 device can be bound to a socket by either calling ax25_bind() or setting SO_BINDTODEVICE socket option. In both cases the refcounts should be incremented, but in fact it is done only in ax25_bind().

This bug leads to the following issue reported by Syzkaller:

================================================================ refcount_t: decrement hit 0; leaking memory. WARNING: CPU: 1 PID: 5932 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31 Modules linked in: CPU: 1 UID: 0 PID: 5932 Comm: syz-executor424 Not tainted 6.13.0-rc4-syzkaller-00110-g4099a71718b0 #0 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014 RIP: 0010:refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31 Call Trace: <TASK> __refcount_dec include/linux/refcount.h:336 [inline] refcount_dec include/linux/refcount.h:351 [inline] ref_tracker_free+0x710/0x820 lib/ref_tracker.c:236 netdev_tracker_free include/linux/netdevice.h:4156 [inline] netdev_put include/linux/netdevice.h:4173 [inline] netdev_put include/linux/netdevice.h:4169 [inline] ax25_release+0x33f/0xa10 net/ax25/af_ax25.c:1069 __sock_release+0xb0/0x270 net/socket.c:640 sock_close+0x1c/0x30 net/socket.c:1408 ... do_syscall_x64 arch/x86/entry/common.c:52 [inline] do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83 entry_SYSCALL_64_after_hwframe+0x77/0x7f ... </TASK>

Fix the implementation of ax25_setsockopt() by adding increment of refcounts for the new device bound, and decrement of refcounts for the old unbound device.

This vulnerability is rated as Moderate severity because it requires local access with user privileges to manipulate AX.25 socket options, leading to a reference count underflow and resource leakage that may cause a denial of service without compromising system confidentiality or integrity. AX.25 is an amateur radio protocol that is not used in standard enterprise deployments, and the module is not loaded by default on Red Hat Enterprise Linux. Consequently, default installations are not exposed unless AX.25 networking is explicitly configured.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.