Subject: Re: [PATCH] slip: Fix use-after-free Read in slip_open

From: [email-masked] (Jouni Högander)
Date: Mon, 25 Nov 2019 14:28:14 +0200

[email-masked] writes:

> From: Jouni Hogander <[email-masked]>
>
> Slip_open doesn't clean-up device which registration failed from the
> slip_devs device list. On next open after failure this list is iterated
> and freed device is accessed. Fix this by calling sl_free_netdev in error
> path.
>
> Here is the trace from the Syzbot:
>
> __dump_stack lib/dump_stack.c:77 [inline]
> dump_stack+0x197/0x210 lib/dump_stack.c:118
> print_address_description.constprop.0.cold+0xd4/0x30b mm/kasan/report.c:374
> __kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
> kasan_report+0x12/0x20 mm/kasan/common.c:634
> __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:132
> sl_sync drivers/net/slip/slip.c:725 [inline]
> slip_open+0xecd/0x11b7 drivers/net/slip/slip.c:801
> tty_ldisc_open.isra.0+0xa3/0x110 drivers/tty/tty_ldisc.c:469
> tty_set_ldisc+0x30e/0x6b0 drivers/tty/tty_ldisc.c:596
> tiocsetd drivers/tty/tty_io.c:2334 [inline]
> tty_ioctl+0xe8d/0x14f0 drivers/tty/tty_io.c:2594
> vfs_ioctl fs/ioctl.c:46 [inline]
> file_ioctl fs/ioctl.c:509 [inline]
> do_vfs_ioctl+0xdb6/0x13e0 fs/ioctl.c:696
> ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
> __do_sys_ioctl fs/ioctl.c:720 [inline]
> __se_sys_ioctl fs/ioctl.c:718 [inline]
> __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
> do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Fixes: 3b5a39979daf ("slip: Fix memory leak in slip_open error path")
> Reported-by: [email-masked]

Unfortunately I'm not able to reproduce this by myself. But to my
understanding scenario described above is possible path to this
use-after-free bug.

> Cc: David Miller <[email-masked]>
> Cc: Oliver Hartkopp <[email-masked]>
> Cc: Lukas Bulwahn <[email-masked]>
> Signed-off-by: Jouni Hogander <[email-masked]>
> ---
>  drivers/net/slip/slip.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/drivers/net/slip/slip.c b/drivers/net/slip/slip.c
> index 4d479e3c817d..2a91c192659f 100644
> --- a/drivers/net/slip/slip.c
> +++ b/drivers/net/slip/slip.c
> @@ -855,6 +855,7 @@ static int slip_open(struct tty_struct *tty)
>  	sl->tty = NULL;
>  	tty->disc_data = NULL;
>  	clear_bit(SLF_INUSE, &sl->flags);
> +	sl_free_netdev(sl->dev);
>  	free_netdev(sl->dev);
>  
>  err_exit:

BR,

Jouni Högander