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Message-ID: <2026020421-wrist-cupbearer-2a2b@gregkh>
Date: Wed, 4 Feb 2026 09:30:45 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>
Cc: netdev@...r.kernel.org, Jiayuan Chen <jiayuan.chen@...pee.com>,
syzbot+827272712bd6d12c79a4@...kaller.appspotmail.com,
Andrew Lunn <andrew+netdev@...n.ch>,
"David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
Simon Horman <horms@...nel.org>,
"Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
Sjur Braendeland <sjur.brandeland@...ricsson.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH net v1] serial: caif: fix use-after-free in caif_serial
ldisc_close()
On Wed, Feb 04, 2026 at 04:19:33PM +0800, Jiayuan Chen wrote:
> From: Jiayuan Chen <jiayuan.chen@...pee.com>
>
> There is a use-after-free bug in caif_serial where handle_tx() may
> access ser->tty after the tty has been freed.
>
> The race condition occurs between ldisc_close() and packet transmission:
>
> CPU 0 (close) CPU 1 (xmit)
> ------------- ------------
> ldisc_close()
> tty_kref_put(ser->tty)
> [tty may be freed here]
> <-- race window -->
> caif_xmit()
> handle_tx()
> tty = ser->tty // dangling ptr
> tty->ops->write() // UAF!
> schedule_work()
> ser_release()
> unregister_netdevice()
>
> The root cause is that tty_kref_put() is called in ldisc_close() while
> the network device is still active and can receive packets.
>
> Since ser and tty have a 1:1 binding relationship with consistent
> lifecycles (ser is created/destroyed in ldisc_open/close, and each ser
> binds exactly one tty), we can safely defer the tty reference release
> to ser_release() where the network device is unregistered.
>
> Fix this by moving tty_kref_put() from ldisc_close() to ser_release(),
> after unregister_netdevice(). This ensures the tty reference is held
> as long as the network device exists, preventing the UAF.
>
> Note: We save ser->tty before unregister_netdevice() because ser is
> embedded in netdev's private data and will be freed along with netdev
> (needs_free_netdev = true).
>
> How to reproduce: Add mdelay(500) at the beginning of ldisc_close()
> to widen the race window, then run the reproducer program [1].
>
> Note: There is a separate deadloop issue in handle_tx() when using
> PORT_UNKNOWN serial ports (e.g., /dev/ttyS3 in QEMU without proper
> serial backend). This deadloop exists even without this patch,
> and is caused by an independent bug: inconsistency between
> uart_write_room() and uart_write() in serial core. It has been
> addressed in a separate patch [2].
>
> KASAN report:
>
> ==================================================================
> BUG: KASAN: slab-use-after-free in handle_tx+0x5d1/0x620
> Read of size 1 at addr ffff8881131e1490 by task caif_uaf_trigge/9929
>
> Call Trace:
> <TASK>
> dump_stack_lvl+0x10e/0x1f0
> print_report+0xd0/0x630
> kasan_report+0xe4/0x120
> handle_tx+0x5d1/0x620
> dev_hard_start_xmit+0x9d/0x6c0
> __dev_queue_xmit+0x6e2/0x4410
> packet_xmit+0x243/0x360
> packet_sendmsg+0x26cf/0x5500
> __sys_sendto+0x4a3/0x520
> __x64_sys_sendto+0xe0/0x1c0
> do_syscall_64+0xc9/0xf80
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f615df2c0d7
>
> Allocated by task 9930:
>
> Freed by task 64:
>
> Last potentially related work creation:
>
> The buggy address belongs to the object at ffff8881131e1000
> which belongs to the cache kmalloc-cg-2k of size 2048
> The buggy address is located 1168 bytes inside of
> freed 2048-byte region [ffff8881131e1000, ffff8881131e1800)
>
> The buggy address belongs to the physical page:
> page_owner tracks the page as allocated
> page last free pid 9778 tgid 9778 stack trace:
>
> Memory state around the buggy address:
> ffff8881131e1380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff8881131e1400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> >ffff8881131e1480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ^
> ffff8881131e1500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ffff8881131e1580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
> ==================================================================
> [1]: https://gist.github.com/mrpre/f683f244544f7b11e7fa87df9e6c2eeb
> [2]: https://lore.kernel.org/linux-serial/20260204074327.226165-1-jiayuan.chen@linux.dev/T/#u
>
> Reported-by: syzbot+827272712bd6d12c79a4@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/000000000000a4a7550611e234f5@google.com/T/
> Fixes: 9b27105b4a44 ("net-caif-driver: add CAIF serial driver (ldisc)")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@...pee.com>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@...ux.dev>
> ---
> drivers/net/caif/caif_serial.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/net/caif/caif_serial.c b/drivers/net/caif/caif_serial.c
> index c398ac42eae9..b90890030751 100644
> --- a/drivers/net/caif/caif_serial.c
> +++ b/drivers/net/caif/caif_serial.c
> @@ -284,6 +284,7 @@ static void ser_release(struct work_struct *work)
> {
> struct list_head list;
> struct ser_device *ser, *tmp;
> + struct tty_struct *tty;
>
> spin_lock(&ser_lock);
> list_replace_init(&ser_release_list, &list);
> @@ -292,9 +293,11 @@ static void ser_release(struct work_struct *work)
> if (!list_empty(&list)) {
> rtnl_lock();
> list_for_each_entry_safe(ser, tmp, &list, node) {
> + tty = ser->tty;
> dev_close(ser->dev);
> unregister_netdevice(ser->dev);
> debugfs_deinit(ser);
> + tty_kref_put(tty);
> }
> rtnl_unlock();
> }
> @@ -355,8 +358,6 @@ static void ldisc_close(struct tty_struct *tty)
> {
> struct ser_device *ser = tty->disc_data;
>
> - tty_kref_put(ser->tty);
> -
Good catch!
While I don't have this hardware to test, this looks sane to me.
Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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