Subject: Re: [PATCH] can: slcan: fix freed work crash

From: Petr Vorel <[email-masked]>
Date: Thu, 1 Dec 2022 11:13:35 +0100

Hi all,

Cc LTP mailing list.
Jiri, thanks a lot for quick fix!

Kind regards,
Petr

> The LTP test pty03 is causing a crash in slcan:
>   BUG: kernel NULL pointer dereference, address: 0000000000000008
>   #PF: supervisor read access in kernel mode
>   #PF: error_code(0x0000) - not-present page
>   PGD 0 P4D 0
>   Oops: 0000 [#1] PREEMPT SMP NOPTI
>   CPU: 0 PID: 348 Comm: kworker/0:3 Not tainted 6.0.8-1-default #1 openSUSE Tumbleweed 9d20364b934f5aab0a9bdf84e8f45cfdfae39dab
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b-rebuilt.opensuse.org 04/01/2014
>   Workqueue:  0x0 (events)
>   RIP: 0010:process_one_work (/home/rich/kernel/linux/kernel/workqueue.c:706 /home/rich/kernel/linux/kernel/workqueue.c:2185)
>   Code: 49 89 ff 41 56 41 55 41 54 55 53 48 89 f3 48 83 ec 10 48 8b 06 48 8b 6f 48 49 89 c4 45 30 e4 a8 04 b8 00 00 00 00 4c 0f 44 e0 <49> 8b 44 24 08 44 8b a8 00 01 00 00 41 83 e5 20 f6 45 10 04 75 0e
>   RSP: 0018:ffffaf7b40f47e98 EFLAGS: 00010046
>   RAX: 0000000000000000 RBX: ffff9d644e1b8b48 RCX: ffff9d649e439968
>   RDX: 00000000ffff8455 RSI: ffff9d644e1b8b48 RDI: ffff9d64764aa6c0
>   RBP: ffff9d649e4335c0 R08: 0000000000000c00 R09: ffff9d64764aa734
>   R10: 0000000000000007 R11: 0000000000000001 R12: 0000000000000000
>   R13: ffff9d649e4335e8 R14: ffff9d64490da780 R15: ffff9d64764aa6c0
>   FS:  0000000000000000(0000) GS:ffff9d649e400000(0000) knlGS:0000000000000000
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 0000000000000008 CR3: 0000000036424000 CR4: 00000000000006f0
>   Call Trace:
>    <TASK>
>   worker_thread (/home/rich/kernel/linux/kernel/workqueue.c:2436)
>   kthread (/home/rich/kernel/linux/kernel/kthread.c:376)
>   ret_from_fork (/home/rich/kernel/linux/arch/x86/entry/entry_64.S:312)

> Apparently, the slcan's tx_work is freed while being scheduled. While
> slcan_netdev_close() (netdev side) calls flush_work(&sl->tx_work),
> slcan_close() (tty side) does not. So when the netdev is never set UP,
> but the tty is stuffed with bytes and forced to wakeup write, the work
> is scheduled, but never flushed.

> So add an additional flush_work() to slcan_close() to be sure the work
> is flushed under all circumstances.

> The Fixes commit below moved flush_work() from slcan_close() to
> slcan_netdev_close(). What was the rationale behind it? Maybe we can
> drop the one in slcan_netdev_close()?

> I see the same pattern in can327. So it perhaps needs the very same fix.

> Fixes: cfcb4465e992 ("can: slcan: remove legacy infrastructure")
> Link: https://bugzilla.suse.com/show_bug.cgi?id=1205597
> Reported-by: Richard Palethorpe <[email-masked]>
> Tested-by: Petr Vorel <[email-masked]>
> Cc: Dario Binacchi <[email-masked]>
> Cc: Wolfgang Grandegger <[email-masked]>
> Cc: Marc Kleine-Budde <[email-masked]>
> Cc: "David S. Miller" <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Paolo Abeni <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: [email-masked]
> Cc: Max Staudt <[email-masked]>
> Signed-off-by: Jiri Slaby (SUSE) <[email-masked]>
> ---
>  drivers/net/can/slcan/slcan-core.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)

> diff --git a/drivers/net/can/slcan/slcan-core.c b/drivers/net/can/slcan/slcan-core.c
> index fbb34139daa1..f4db77007c13 100644
> --- a/drivers/net/can/slcan/slcan-core.c
> +++ b/drivers/net/can/slcan/slcan-core.c
> @@ -864,12 +864,14 @@ static void slcan_close(struct tty_struct *tty)
>  {
>  	struct slcan *sl = (struct slcan *)tty->disc_data;

> -	/* unregister_netdev() calls .ndo_stop() so we don't have to.
> -	 * Our .ndo_stop() also flushes the TTY write wakeup handler,
> -	 * so we can safely set sl->tty = NULL after this.
> -	 */
>  	unregister_candev(sl->dev);

> +	/*
> +	 * The netdev needn't be UP (so .ndo_stop() is not called). Hence make
> +	 * sure this is not running before freeing it up.
> +	 */
> +	flush_work(&sl->tx_work);
> +
>  	/* Mark channel as dead */
>  	spin_lock_bh(&sl->lock);
>  	tty->disc_data = NULL;