[v5.15] INFO: task hung in __fdget_pos (4)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 98f47d0e9b8c Linux 5.15.184
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=118c0c82580000
kernel config: https://syzkaller.appspot.com/x/.config?x=6d966a9f787a9782
dashboard link: https://syzkaller.appspot.com/bug?extid=229b5abd47623496cc52
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=138c0c82580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16eed00c580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5e51314fa9b0/disk-98f47d0e.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3348ed80ea4e/vmlinux-98f47d0e.xz
kernel image: https://storage.googleapis.com/syzbot-assets/3e02a814f2cc/bzImage-98f47d0e.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+229b5a...@syzkaller.appspotmail.com

INFO: task syz-executor980:4169 blocked for more than 143 seconds.
Not tainted 5.15.184-syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor980 state:D stack:29448 pid: 4169 ppid: 4165 flags:0x00004006
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5030 [inline]
__schedule+0x11b8/0x43b0 kernel/sched/core.c:6376
schedule+0x11b/0x1e0 kernel/sched/core.c:6459
schedule_preempt_disabled+0xf/0x20 kernel/sched/core.c:6518
__mutex_lock_common+0xc71/0x2390 kernel/locking/mutex.c:669
__mutex_lock kernel/locking/mutex.c:729 [inline]
mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
__fdget_pos+0x2bf/0x370 fs/file.c:1058
fdget_pos include/linux/file.h:75 [inline]
__do_sys_getdents64 fs/readdir.c:365 [inline]
__se_sys_getdents64+0xbe/0x260 fs/readdir.c:354
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f3b366e6869
RSP: 002b:00007f3b36686218 EFLAGS: 00000246 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 00007f3b36770418 RCX: 00007f3b366e6869
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000006
RBP: 00007f3b36770410 R08: 00007ffe0d789157 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f3b3673d07c
R13: 0030656c69662f2e R14: 00007ffe0d789070 R15: 0000200000004280
</TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/27:
#0: ffffffff8c11c060 (rcu_read_lock){....}-{1:2}, at: rcu_lock_acquire+0x0/0x30
2 locks held by getty/3933:
#0: ffff88802a7c3098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x21/0x70 drivers/tty/tty_ldisc.c:252
#1: ffffc900025c62e8 (&ldata->atomic_read_lock){+.+.}-{3:3}, at: n_tty_read+0x5ba/0x1a30 drivers/tty/n_tty.c:2158
2 locks held by syz-executor980/4167:
#0: ffff88802a416370 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0x2bf/0x370 fs/file.c:1058
#1: ffff888074550298 (&sb->s_type->i_mutex_key#15){.+.+}-{3:3}, at: iterate_dir+0x10d/0x560 fs/readdir.c:55
1 lock held by syz-executor980/4169:
#0: ffff88802a416370 (&f->f_pos_lock){+.+.}-{3:3}, at: __fdget_pos+0x2bf/0x370 fs/file.c:1058

=============================================

NMI backtrace for cpu 0
CPU: 0 PID: 27 Comm: khungtaskd Not tainted 5.15.184-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
<TASK>
dump_stack_lvl+0x168/0x230 lib/dump_stack.c:106
nmi_cpu_backtrace+0x397/0x3d0 lib/nmi_backtrace.c:111
nmi_trigger_cpumask_backtrace+0x163/0x280 lib/nmi_backtrace.c:62
trigger_all_cpu_backtrace include/linux/nmi.h:148 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:212 [inline]
watchdog+0xe0f/0xe50 kernel/hung_task.c:369
kthread+0x436/0x520 kernel/kthread.c:334
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287
</TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1 skipped: idling at native_safe_halt arch/x86/include/asm/irqflags.h:51 [inline]
NMI backtrace for cpu 1 skipped: idling at arch_safe_halt arch/x86/include/asm/irqflags.h:89 [inline]
NMI backtrace for cpu 1 skipped: idling at default_idle+0xb/0x10 arch/x86/kernel/process.c:722


---
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[syzbot]

syzbot suspects this issue could be fixed by backporting the following commit:

commit fe1371d0f8c6a9aa4441ddd09ea6d4f31b975a81
git tree: upstream
Author: Eric Van Hensbergen <eri...@kernel.org>
Date: Fri Jan 5 16:16:36 2024 +0000

fs/9p: switch vfsmount to use v9fs_get_new_inode

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=14514982580000

kernel config: https://syzkaller.appspot.com/x/.config?x=6d966a9f787a9782
dashboard link: https://syzkaller.appspot.com/bug?extid=229b5abd47623496cc52

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=138c0c82580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16eed00c580000

Please keep in mind that other backports might be required as well.

For information about bisection process see: https://goo.gl/tpsmEJ#bisection