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Message-ID: <20260204081939.237738-1-jiayuan.chen@linux.dev>
Date: Wed, 4 Feb 2026 16:19:33 +0800
From: Jiayuan Chen <jiayuan.chen@...ux.dev>
To: netdev@...r.kernel.org
Cc: Jiayuan Chen <jiayuan.chen@...pee.com>,
syzbot+827272712bd6d12c79a4@...kaller.appspotmail.com,
Jiayuan Chen <jiayuan.chen@...ux.dev>,
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>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
"Jiri Slaby (SUSE)" <jirislaby@...nel.org>,
Sjur Braendeland <sjur.brandeland@...ricsson.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH net v1] serial: caif: fix use-after-free in caif_serial ldisc_close()
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);
-
spin_lock(&ser_lock);
list_move(&ser->node, &ser_release_list);
spin_unlock(&ser_lock);
--
2.43.0
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