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Message-ID: <2b99c5ad-c6f6-4f24-9a62-e24d67d7a145@huawei.com>
Date: Mon, 9 Feb 2026 15:51:14 +0800
From: Jijie Shao <shaojijie@...wei.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>, <netdev@...r.kernel.org>
CC: <shaojijie@...wei.com>, Jiayuan Chen <jiayuan.chen@...pee.com>,
<syzbot+827272712bd6d12c79a4@...kaller.appspotmail.com>, Greg Kroah-Hartman
<gregkh@...uxfoundation.org>, 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>, Konstantin
Khlebnikov <khlebnikov@...nvz.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net v2] serial: caif: fix use-after-free in caif_serial
ldisc_close()
on 2026/2/6 15:44, 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 allocated in ldisc_open and freed in ser_release
> via unregister_netdevice, 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 likely caused by 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: 56e0ef527b18 ("drivers/net: caif: fix wrong rtnl_is_locked() usage")
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@...pee.com>
>
> ---
> v1 -> v2:
> Add Reviewed-by tag.
> Use correct Fixes tag.
> v1: https://lore.kernel.org/netdev/20260204081939.237738-1-jiayuan.chen@linux.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);
from log message:
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).
...
Therefore, since "ser" has already been released, "debugfs_deinit(ser)" should not be at this location.
Of course, this issue is not related to this patch itself.
...
Also, after calling "dev_close(ser->dev)", can we directly call tty_kref_put()?
At this point, there should be no more packets to receive, so you don't need to save "ser->tty" anymore.
Of course, the current changes also look good to me. so,
Reviewed-by: Jijie Shao <shaojijie@...wei.com>
> + 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);
> -
> spin_lock(&ser_lock);
> list_move(&ser->node, &ser_release_list);
> spin_unlock(&ser_lock);
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