linux-kernel - Re: [syzbot] [fs?] KCSAN: data-race in __ep_remove / _

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Message-ID: <CACT4Y+Z34Mc_Vv8-dDvD+HE57tMez0Rd17zr_s-3Gn23tOVG3A@mail.gmail.com>
Date: Thu, 4 Jul 2024 16:40:40 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: syzbot <syzbot+3195ed1f3a2ab8bea49a@...kaller.appspotmail.com>
Cc: brauner@...nel.org, jack@...e.cz, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	viro@...iv.linux.org.uk
Subject: Re: [syzbot] [fs?] KCSAN: data-race in __ep_remove / __fput (4)

On Thu, 4 Jul 2024 at 16:38, syzbot
<syzbot+3195ed1f3a2ab8bea49a@...kaller.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@...kaller.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.


> ==================================================================
> 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
> ==================================================================
>
>
> ---
> 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 syzkaller@...glegroups.com.
>
> syzbot will keep track of this issue. See:
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>
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