Re: [syzbot] [fs?] KCSAN: data-race in __ep_remove / __fput (4)

[Christian Brauner <brauner@kernel.org>]

On Thu, Jul 04, 2024 at 04:40:40PM GMT, Dmitry Vyukov wrote:
> On Thu, 4 Jul 2024 at 16:38, syzbot
> <syzbot+3195ed1f3a2ab8bea49a@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    22a40d14b572 Linux 6.10-rc6
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=10f94dae980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=5b9537cd00be479e
> > dashboard link: https://syzkaller.appspot.com/bug?extid=3195ed1f3a2ab8bea49a
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> >
> > Unfortunately, I don't have any reproducer for this issue yet.
> >
> > Downloadable assets:
> > disk image: https://storage.googleapis.com/syzbot-assets/ebe2f3933faf/disk-22a40d14.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/7227032da0fe/vmlinux-22a40d14.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/a330dc1e107b/bzImage-22a40d14.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+3195ed1f3a2ab8bea49a@syzkaller.appspotmail.com
> 
> 
> Double-checked locking in eventpoll_release_file() should prevent NULL
> derefs in eventpoll_release_file(), right? If so, it's probably
> benign-ish.

Yes, see the comment to eventpoll_release(). I assume a READ_ONCE()
would be ok to add there so that race is annotated.

> 
> 
> > ==================================================================
> > BUG: KCSAN: data-race in __ep_remove / __fput
> >
> > write to 0xffff88810f2358d0 of 8 bytes by task 6036 on cpu 1:
> >  __ep_remove+0x3c9/0x450 fs/eventpoll.c:826
> >  ep_remove_safe fs/eventpoll.c:864 [inline]
> >  ep_clear_and_put+0x158/0x260 fs/eventpoll.c:900
> >  ep_eventpoll_release+0x32/0x50 fs/eventpoll.c:937
> >  __fput+0x2c2/0x660 fs/file_table.c:422
> >  ____fput+0x15/0x20 fs/file_table.c:450
> >  task_work_run+0x13a/0x1a0 kernel/task_work.c:180
> >  resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
> >  exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
> >  __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
> >  syscall_exit_to_user_mode+0xbe/0x130 kernel/entry/common.c:218
> >  do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > read to 0xffff88810f2358d0 of 8 bytes by task 6037 on cpu 0:
> >  eventpoll_release include/linux/eventpoll.h:45 [inline]
> >  __fput+0x234/0x660 fs/file_table.c:413
> >  ____fput+0x15/0x20 fs/file_table.c:450
> >  task_work_run+0x13a/0x1a0 kernel/task_work.c:180
> >  resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
> >  exit_to_user_mode_loop kernel/entry/common.c:114 [inline]
> >  exit_to_user_mode_prepare include/linux/entry-common.h:328 [inline]
> >  __syscall_exit_to_user_mode_work kernel/entry/common.c:207 [inline]
> >  syscall_exit_to_user_mode+0xbe/0x130 kernel/entry/common.c:218
> >  do_syscall_64+0xd6/0x1c0 arch/x86/entry/common.c:89
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > value changed: 0xffff888102f1e010 -> 0x0000000000000000
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 0 PID: 6037 Comm: syz.0.1032 Not tainted 6.10.0-rc6-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/07/2024
> > ==================================================================
> >
> >
> > ---
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