Re: [BUG] WARNING: refcount bug in gsm_mux_net_start_xmit

[Greg Kroah-Hartman]

On Fri, Aug 28, 2026 at 05:53:36AM +0000, Farhad Alemi wrote:
> Hello,
> 
> As part of the kernel research at ASU's SEFCOM
> lab, we hit the crash below. Crash reports can be found here:
> 
>   https://github.com/farhad-alemi/public_bug_reports/tree/main/199-warning-refcount-bug-in-gsm-mux-net-start-xmit/
> 
>   refcount_t: addition on 0; use-after-free.
>   WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110
> lib/refcount.c:25, CPU#0: kworker/u8:1/14
>   Workqueue: ipv6_addrconf addrconf_dad_work
>   Call Trace:
>    refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
>    __refcount_inc include/linux/refcount.h:366 [inline]
>    refcount_inc include/linux/refcount.h:383 [inline]
>    kref_get include/linux/kref.h:45 [inline]
>    muxnet_get drivers/tty/n_gsm.c:3938 [inline]
>    gsm_mux_net_start_xmit+0x3af/0x440 drivers/tty/n_gsm.c:3951
>    __netdev_start_xmit include/linux/netdevice.h:5368 [inline]
>    dev_hard_start_xmit+0x23b/0x620 net/core/dev.c:3904
>    __dev_queue_xmit+0x160b/0x3250 net/core/dev.c:4831
>    ip6_finish_output2+0xfd4/0x19b0 net/ipv6/ip6_output.c:136
>    mld_sendpack+0x820/0xe00 net/ipv6/mcast.c:1855
>    ipv6_mc_dad_complete+0x85/0x470 net/ipv6/mcast.c:2279
> 
> Our reproducer.c is available upon request.
> 
> Happy to test a patch if that would help.

Again, please sned such patch.