This is a note to let you know that I've just added the patch titled
hamradio: defer ax25 kfree after unregister_netdev
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-defer-ax25-kfree-after-unregister_netdev.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 3e0588c291d6ce225f2b891753ca41d45ba42469 Mon Sep 17 00:00:00 2001
From: Lin Ma <linma@xxxxxxxxxx>
Date: Mon, 8 Nov 2021 18:37:21 +0800
Subject: hamradio: defer ax25 kfree after unregister_netdev
From: Lin Ma <linma@xxxxxxxxxx>
commit 3e0588c291d6ce225f2b891753ca41d45ba42469 upstream.
There is a possible race condition (use-after-free) like below
(USE) | (FREE)
ax25_sendmsg |
ax25_queue_xmit |
dev_queue_xmit |
__dev_queue_xmit |
__dev_xmit_skb |
sch_direct_xmit | ...
xmit_one |
netdev_start_xmit | tty_ldisc_kill
__netdev_start_xmit | mkiss_close
ax_xmit | kfree
ax_encaps |
|
Even though there are two synchronization primitives before the kfree:
1. wait_for_completion(&ax->dead). This can prevent the race with
routines from mkiss_ioctl. However, it cannot stop the routine coming
from upper layer, i.e., the ax25_sendmsg.
2. netif_stop_queue(ax->dev). It seems that this line of code aims to
halt the transmit queue but it fails to stop the routine that already
being xmit.
This patch reorder the kfree after the unregister_netdev to avoid the
possible UAF as the unregister_netdev() is well synchronized and won't
return if there is a running routine.
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 | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
--- a/drivers/net/hamradio/mkiss.c
+++ b/drivers/net/hamradio/mkiss.c
@@ -803,13 +803,14 @@ static void mkiss_close(struct tty_struc
*/
netif_stop_queue(ax->dev);
- /* Free all AX25 frame buffers. */
- kfree(ax->rbuff);
- kfree(ax->xbuff);
-
ax->tty = NULL;
unregister_netdev(ax->dev);
+
+ /* Free all AX25 frame buffers after unreg. */
+ kfree(ax->rbuff);
+ kfree(ax->xbuff);
+
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
