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SubjectRe: INFO: task hung in fanotify_handle_event
Hello,

On Mon 15-10-18 04:32:02, syzbot wrote:
> syzbot found the following crash on:
>
> HEAD commit: 90ad18418c2d Merge git://git.kernel.org/pub/scm/linux/kern..
> git tree: upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=17f1776e400000
> kernel config: https://syzkaller.appspot.com/x/.config?x=88e9a8a39dc0be2d
> dashboard link: https://syzkaller.appspot.com/bug?extid=29143581b0ded3213e99
> compiler: gcc (GCC) 8.0.1 20180413 (experimental)
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=123459d6400000
>
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+29143581b0ded3213e99@syzkaller.appspotmail.com

Syzbot has apparently generated fanotify watch for FAN_OPEN_PERM event and
then the process got stuck waiting for userspace to respond to that event -
which never happened. So everything works as designed here - the process
placing FAN_OPEN_PERM mark is responsible for replying to the generated
events as all opens hang waiting for responses. That's why the
functionality is behind CAP_SYS_ADMIN after all... Could we fix syzbot to
actually generate replies for these events?

Honza
>
> INFO: task syz-executor3:23598 blocked for more than 140 seconds.
> Not tainted 4.19.0-rc7+ #59
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> syz-executor3 D24888 23598 5550 0x00000004
> Call Trace:
> context_switch kernel/sched/core.c:2825 [inline]
> __schedule+0x86c/0x1ed0 kernel/sched/core.c:3473
> schedule+0xfe/0x460 kernel/sched/core.c:3517
> fanotify_get_response fs/notify/fanotify/fanotify.c:68 [inline]
> fanotify_handle_event+0x7fb/0x9a0 fs/notify/fanotify/fanotify.c:245
> send_to_group fs/notify/fsnotify.c:234 [inline]
> fsnotify+0x87f/0x12f0 fs/notify/fsnotify.c:367
> fsnotify_perm include/linux/fsnotify.h:52 [inline]
> security_file_open+0x16f/0x1b0 security/security.c:986
> do_dentry_open+0x331/0x1250 fs/open.c:758
> vfs_open+0xa0/0xd0 fs/open.c:880
> do_last fs/namei.c:3418 [inline]
> path_openat+0x12bf/0x5160 fs/namei.c:3534
> do_filp_open+0x255/0x380 fs/namei.c:3564
> do_sys_open+0x568/0x700 fs/open.c:1063
> ksys_open include/linux/syscalls.h:1276 [inline]
> __do_sys_creat fs/open.c:1121 [inline]
> __se_sys_creat fs/open.c:1119 [inline]
> __x64_sys_creat+0x61/0x80 fs/open.c:1119
> do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x457569
> Code: Bad RIP value.
> RSP: 002b:00007efe2663bc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000055
> RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 0000000000457569
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020000180
> RBP: 000000000072bfa0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 00007efe2663c6d4
> R13: 00000000004bdb2f R14: 00000000004cc688 R15: 00000000ffffffff
>
> Showing all locks held in the system:
> 1 lock held by khungtaskd/984:
> #0: 0000000045bbc556 (rcu_read_lock){....}, at:
> debug_show_all_locks+0xd0/0x424 kernel/locking/lockdep.c:4435
> 1 lock held by rsyslogd/5369:
> #0: 00000000c6b46b4b (&f->f_pos_lock){+.+.}, at: __fdget_pos+0x1bb/0x200
> fs/file.c:766
> 2 locks held by getty/5491:
> #0: 000000007c8f39bf (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 00000000ab767e83 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5492:
> #0: 000000005a020001 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 000000001a016d07 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5493:
> #0: 00000000317a902c (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 00000000de804861 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5494:
> #0: 00000000bd67aa3a (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 00000000c6fa2e6f (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5495:
> #0: 00000000d69ad6b3 (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 00000000b28afab5 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5496:
> #0: 00000000f21d8abe (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 00000000ed3e038f (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 2 locks held by getty/5497:
> #0: 000000005843227c (&tty->ldisc_sem){++++}, at: ldsem_down_read+0x32/0x40
> drivers/tty/tty_ldsem.c:353
> #1: 000000005ec4a201 (&ldata->atomic_read_lock){+.+.}, at:
> n_tty_read+0x335/0x1ce0 drivers/tty/n_tty.c:2140
> 1 lock held by syz-executor3/23598:
> #0: 0000000008161dc8 (sb_writers#4){.+.+}, at: sb_start_write
> include/linux/fs.h:1566 [inline]
> #0: 0000000008161dc8 (sb_writers#4){.+.+}, at: mnt_want_write+0x3f/0xc0
> fs/namespace.c:360
>
> =============================================
>
> NMI backtrace for cpu 0
> CPU: 0 PID: 984 Comm: khungtaskd Not tainted 4.19.0-rc7+ #59
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:77 [inline]
> dump_stack+0x1c4/0x2b4 lib/dump_stack.c:113
> nmi_cpu_backtrace.cold.3+0x63/0xa2 lib/nmi_backtrace.c:101
> nmi_trigger_cpumask_backtrace+0x1b3/0x1ed lib/nmi_backtrace.c:62
> arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
> trigger_all_cpu_backtrace include/linux/nmi.h:144 [inline]
> check_hung_uninterruptible_tasks kernel/hung_task.c:204 [inline]
> watchdog+0xb3e/0x1050 kernel/hung_task.c:265
> kthread+0x35a/0x420 kernel/kthread.c:246
> ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:413
> Sending NMI from CPU 0 to CPUs 1:
> NMI backtrace for cpu 1 skipped: idling at native_safe_halt+0x6/0x10
> arch/x86/include/asm/irqflags.h:57
>
>
> ---
> This bug 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 syzkaller@googlegroups.com.
>
> syzbot will keep track of this bug report. See:
> https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with
> syzbot.
> syzbot can test patches for this bug, for details see:
> https://goo.gl/tpsmEJ#testing-patches
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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