[PATCH 5.4 46/47] hamradio: improve the incomplete fix to avoid NPD

Greg Kroah-Hartman
2021-12-27T15:37:27Z

From: Lin Ma <linma@zju.edu.cn>

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@zju.edu.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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
@@ -793,14 +793,14 @@ 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);
}