Re: [PATCH] net: Fix potential NULL pointer dereference in __skb_try_recv_datagram

Eric Dumazet [off-list ref] writes:

On Mon, 2016-01-04 at 10:10 +0100, Jacob Siverskog wrote:

[...]

Hmm ... the code continues with

		err = tty_unregister_ldisc(N_TI_WL);
		if (err)
			pr_err("unable to un-register ldisc %ld", err);
		/* free the global data pointer */
		kfree(st_gdata);

So who would ever see that the rx_skb and tx_skb pointers were cleared
prior to freeing the data structure containing them?

On Mon, 2016-01-04 at 16:14 +0000, Rainer Weikusat wrote:

Eric Dumazet [off-list ref] writes:

quoted

On Mon, 2016-01-04 at 10:10 +0100, Jacob Siverskog wrote:

[...]

quoted

quoted

I believe the crash occurred between these two actions. I just saw
that there are some interesting events in the log prior to the crash:
kernel: Bluetooth: Unable to push skb to HCI core(-6)
kernel: (stc):  proto stack 4's ->recv failed
kernel: (stc): remove_channel_from_table: id 3
kernel: (stc): remove_channel_from_table: id 2
kernel: (stc): remove_channel_from_table: id 4
kernel: (stc):  all chnl_ids unregistered
kernel: (stk) :ldisc_install = 0(stc): st_tty_close

The first print is from btwilink.c. However, I can't see the
connection between Bluetooth (BLE) and UDP/IPv6 (we're not using
6LoWPAN or anything similar).

Thanks, Jacob

Definitely these details are useful ;)

Could you try :

diff --git a/drivers/misc/ti-st/st_core.c b/drivers/misc/ti-st/st_core.c
index 6e3af8b42cdd..0c99a74fb895 100644
--- a/drivers/misc/ti-st/st_core.c
+++ b/drivers/misc/ti-st/st_core.c

@@ -912,7 +912,9 @@ void st_core_exit(struct st_data_s *st_gdata)
 		skb_queue_purge(&st_gdata->txq);
 		skb_queue_purge(&st_gdata->tx_waitq);
 		kfree_skb(st_gdata->rx_skb);
+		st_gdata->rx_skb = NULL;
 		kfree_skb(st_gdata->tx_skb);
+		st_gdata->tx_skb = NULL;
 		/* TTY ldisc cleanup */
 		err = tty_unregister_ldisc(N_TI_WL);
 		if (err)

Hmm ... the code continues with

		err = tty_unregister_ldisc(N_TI_WL);
		if (err)
			pr_err("unable to un-register ldisc %ld", err);
		/* free the global data pointer */
		kfree(st_gdata);

So who would ever see that the rx_skb and tx_skb pointers were cleared
prior to freeing the data structure containing them?

This is the theory, but I suspect a use after free.

kfree(st_gdata) does not clear all content with 0, unless you use
special SLUB/SLAB debugging features.