[syzbot] [ppp?] KMSAN: uninit-value in ppp_sync_receive (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9a9c8ce300cd Merge tag 'kbuild-fixes-7.0-4' of git://git.k..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=112a874e580000
kernel config: https://syzkaller.appspot.com/x/.config?x=963de479f54c6dbb
dashboard link: https://syzkaller.appspot.com/bug?extid=88679c919eb801bd16f8
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/3e2ad23d804a/disk-9a9c8ce3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/a3dde4990296/vmlinux-9a9c8ce3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/256e0a89d22c/bzImage-9a9c8ce3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+88679c...@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in ppp_sync_input drivers/net/ppp/ppp_synctty.c:684 [inline]
BUG: KMSAN: uninit-value in ppp_sync_receive+0x626/0xfa0 drivers/net/ppp/ppp_synctty.c:334
ppp_sync_input drivers/net/ppp/ppp_synctty.c:684 [inline]
ppp_sync_receive+0x626/0xfa0 drivers/net/ppp/ppp_synctty.c:334
tty_ldisc_receive_buf+0x1f7/0x2c0 drivers/tty/tty_buffer.c:391
tty_port_default_receive_buf+0xd7/0x1a0 drivers/tty/tty_port.c:37
receive_buf drivers/tty/tty_buffer.c:445 [inline]
flush_to_ldisc+0x43e/0xe40 drivers/tty/tty_buffer.c:495
process_one_work kernel/workqueue.c:3276 [inline]
process_scheduled_works+0xb82/0x1e80 kernel/workqueue.c:3359
worker_thread+0xee4/0x1590 kernel/workqueue.c:3440
kthread+0x53f/0x600 kernel/kthread.c:436
ret_from_fork+0x20f/0x910 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Uninit was created at:
slab_post_alloc_hook mm/slub.c:4545 [inline]
slab_alloc_node mm/slub.c:4866 [inline]
__do_kmalloc_node mm/slub.c:5259 [inline]
__kmalloc_noprof+0x486/0x1680 mm/slub.c:5272
kmalloc_noprof include/linux/slab.h:954 [inline]
tty_buffer_alloc drivers/tty/tty_buffer.c:180 [inline]
__tty_buffer_request_room+0x3d4/0x7a0 drivers/tty/tty_buffer.c:273
__tty_insert_flip_string_flags+0x157/0x6e0 drivers/tty/tty_buffer.c:309
tty_insert_flip_char include/linux/tty_flip.h:77 [inline]
uart_insert_char+0x368/0x930 drivers/tty/serial/serial_core.c:3431
serial8250_read_char+0x1ba/0x670 drivers/tty/serial/8250/8250_port.c:1643
serial8250_rx_chars drivers/tty/serial/8250/8250_port.c:1660 [inline]
serial8250_handle_irq_locked+0x6d4/0xa40 drivers/tty/serial/8250/8250_port.c:1820
serial8250_handle_irq+0x187/0x730 drivers/tty/serial/8250/8250_port.c:1841
serial8250_default_handle_irq+0x116/0x370 drivers/tty/serial/8250/8250_port.c:1855
serial8250_interrupt+0xcb/0x420 drivers/tty/serial/8250/8250_core.c:86
__handle_irq_event_percpu+0x13c/0xf90 kernel/irq/handle.c:209
handle_irq_event_percpu kernel/irq/handle.c:246 [inline]
handle_irq_event+0xe0/0x2a0 kernel/irq/handle.c:263
handle_edge_irq+0x2a9/0xb30 kernel/irq/chip.c:855
generic_handle_irq_desc include/linux/irqdesc.h:186 [inline]
handle_irq arch/x86/kernel/irq.c:262 [inline]
call_irq_handler arch/x86/kernel/irq.c:-1 [inline]
__common_interrupt+0x9d/0x180 arch/x86/kernel/irq.c:333
common_interrupt+0x4c/0xb0 arch/x86/kernel/irq.c:326
asm_common_interrupt+0x2b/0x40 arch/x86/include/asm/idtentry.h:688

CPU: 0 UID: 0 PID: 35 Comm: kworker/u8:2 Tainted: G W L syzkaller #0 PREEMPT(full)
Tainted: [W]=WARN, [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Workqueue: events_unbound flush_to_ldisc
=====================================================


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup