CVE-2026-45866
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Use-After-Free in Linux Kernel CAIF Serial Driver

Publication date: 2026-05-27

Last updated on: 2026-05-27

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: serial: caif: fix use-after-free in caif_serial ldisc_close() 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/[email protected]/T/#u
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Meta Information
Published
2026-05-27
Last Modified
2026-05-27
Generated
2026-06-16
AI Q&A
2026-05-27
EPSS Evaluated
2026-06-15
NVD
CVE-2026-45866
EUVD
EUVD-2026-32332
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
linux linux_kernel *
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CWE ID Description
CWE-UNKNOWN
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Executive Summary

This vulnerability is a use-after-free bug in the Linux kernel's caif_serial driver. Specifically, the function handle_tx() may access a tty (teletypewriter) pointer after it has already been freed. This happens due to a race condition between the closing of the line discipline (ldisc_close()) and packet transmission (handle_tx()).

The root cause is that the tty reference is released too early in ldisc_close() while the network device is still active and can receive packets. This leads to handle_tx() using a dangling pointer to the tty, causing a use-after-free error.

The fix involves deferring the release of the tty reference until after the network device is unregistered in ser_release(), ensuring the tty remains valid as long as the network device exists.

Impact Analysis

This use-after-free vulnerability can lead to undefined behavior in the kernel, including potential crashes or memory corruption. Since it involves accessing freed memory, it could be exploited to cause denial of service or potentially escalate privileges if an attacker can trigger the race condition.

The race condition occurs during packet transmission and device closure, which may be triggered by network activity, making it a risk in environments using the affected serial driver.

Detection Guidance

This vulnerability involves a use-after-free bug in the Linux kernel's caif_serial driver, specifically in the race condition between ldisc_close() and packet transmission in handle_tx().

To detect this vulnerability on your system, you can attempt to reproduce the race condition by adding a delay in ldisc_close() to widen the race window and then running a reproducer program as described in the CVE details.

The CVE references a reproducer program available at https://gist.github.com/mrpre/f683f244544f7b11e7fa87df9e6c2eeb which can be used to trigger the bug and detect its presence.

No specific detection commands are provided in the available information.

Mitigation Strategies

The root cause of the vulnerability is that tty_kref_put() is called too early in ldisc_close() while the network device is still active, leading to a use-after-free condition.

The fix involves deferring the release of the tty reference to ser_release(), after the network device is unregistered, ensuring the tty reference is held as long as the network device exists.

Immediate mitigation steps include updating the Linux kernel to a version that includes this fix, which moves tty_kref_put() from ldisc_close() to ser_release().

Until the patch is applied, avoid conditions that trigger the race window, such as closing serial line disciplines while packets are being transmitted.

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