This is a note to let you know that I've just added the patch titled hamradio: improve the incomplete fix to avoid NPD to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: hamradio-improve-the-incomplete-fix-to-avoid-npd.patch and it can be found in the queue-4.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From b2f37aead1b82a770c48b5d583f35ec22aabb61e Mon Sep 17 00:00:00 2001 From: Lin Ma <linma@xxxxxxxxxx> Date: Fri, 17 Dec 2021 10:13:56 +0800 Subject: hamradio: improve the incomplete fix to avoid NPD From: Lin Ma <linma@xxxxxxxxxx> commit b2f37aead1b82a770c48b5d583f35ec22aabb61e upstream. The previous commit 3e0588c291d6 ("hamradio: defer ax25 kfree after unregister_netdev") reorder the kfree operations and unregister_netdev operation to prevent UAF. This commit improves the previous one by also deferring the nullify of the ax->tty pointer. Otherwise, a NULL pointer dereference bug occurs. Partial of the stack trace is shown below. BUG: kernel NULL pointer dereference, address: 0000000000000538 RIP: 0010:ax_xmit+0x1f9/0x400 ... Call Trace: dev_hard_start_xmit+0xec/0x320 sch_direct_xmit+0xea/0x240 __qdisc_run+0x166/0x5c0 __dev_queue_xmit+0x2c7/0xaf0 ax25_std_establish_data_link+0x59/0x60 ax25_connect+0x3a0/0x500 ? security_socket_connect+0x2b/0x40 __sys_connect+0x96/0xc0 ? __hrtimer_init+0xc0/0xc0 ? common_nsleep+0x2e/0x50 ? switch_fpu_return+0x139/0x1a0 __x64_sys_connect+0x11/0x20 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xa9 The crash point is shown as below static void ax_encaps(...) { ... set_bit(TTY_DO_WRITE_WAKEUP, &ax->tty->flags); // ax->tty = NULL! ... } By placing the nullify action after the unregister_netdev, the ax->tty pointer won't be assigned as NULL net_device framework layer is well synchronized. Signed-off-by: Lin Ma <linma@xxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- drivers/net/hamradio/mkiss.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- a/drivers/net/hamradio/mkiss.c +++ b/drivers/net/hamradio/mkiss.c @@ -803,14 +803,14 @@ static void mkiss_close(struct tty_struc */ netif_stop_queue(ax->dev); - ax->tty = NULL; - unregister_netdev(ax->dev); /* Free all AX25 frame buffers after unreg. */ kfree(ax->rbuff); kfree(ax->xbuff); + ax->tty = NULL; + free_netdev(ax->dev); } Patches currently in stable-queue which might be from linma@xxxxxxxxxx are queue-4.4/hamradio-improve-the-incomplete-fix-to-avoid-npd.patch queue-4.4/hamradio-defer-ax25-kfree-after-unregister_netdev.patch queue-4.4/ax25-npd-bug-when-detaching-ax25-device.patch