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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 24 Apr 2023 09:37:23 +0200
Message-ID: <CACT4Y+bVUkaoyp5OdzGLipof0b1+ec8xwqS+8cgvObuV0BUc5g@mail.gmail.com>
Subject: Re: [syzbot] [io-uring?] KCSAN: data-race in __io_fill_cqe_req / io_timeout
To:     syzbot <syzbot+cb265db2f3f3468ef436@syzkaller.appspotmail.com>
Cc:     asml.silence@gmail.com, axboe@kernel.dk, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com
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On Mon, 24 Apr 2023 at 09:19, syzbot
<syzbot+cb265db2f3f3468ef436@syzkaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    3a93e40326c8 Merge tag 'for-linus' of git://git.kernel.org..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1280071ec80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f7350c77b8056a38
> dashboard link: https://syzkaller.appspot.com/bug?extid=cb265db2f3f3468ef436
> compiler:       Debian clang version 15.0.7, GNU ld (GNU Binutils for Debian) 2.35.2
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/2122926bc9fe/disk-3a93e403.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/8392992358bc/vmlinux-3a93e403.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/6398a2d19a7e/bzImage-3a93e403.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+cb265db2f3f3468ef436@syzkaller.appspotmail.com

I did not fully grasp what happens here, but it looks suspicious.
The comment in io_timeout() says that it computes "events that need to
occur for this timeout event to be satisfied". But if the range is
racing, it marks a random set of events?


> ==================================================================
> BUG: KCSAN: data-race in __io_fill_cqe_req / io_timeout
>
> read-write to 0xffff888108bf8310 of 4 bytes by task 20447 on cpu 0:
>  io_get_cqe_overflow io_uring/io_uring.h:112 [inline]
>  io_get_cqe io_uring/io_uring.h:124 [inline]
>  __io_fill_cqe_req+0x6c/0x4d0 io_uring/io_uring.h:137
>  io_fill_cqe_req io_uring/io_uring.h:165 [inline]
>  __io_req_complete_post+0x67/0x790 io_uring/io_uring.c:969
>  io_req_complete_post io_uring/io_uring.c:1006 [inline]
>  io_req_task_complete+0xb9/0x110 io_uring/io_uring.c:1654
>  handle_tw_list io_uring/io_uring.c:1184 [inline]
>  tctx_task_work+0x1fe/0x4d0 io_uring/io_uring.c:1246
>  task_work_run+0x123/0x160 kernel/task_work.c:179
>  get_signal+0xe5c/0xfe0 kernel/signal.c:2635
>  arch_do_signal_or_restart+0x89/0x2b0 arch/x86/kernel/signal.c:306
>  exit_to_user_mode_loop+0x6d/0xe0 kernel/entry/common.c:168
>  exit_to_user_mode_prepare+0x6a/0xa0 kernel/entry/common.c:204
>  __syscall_exit_to_user_mode_work kernel/entry/common.c:286 [inline]
>  syscall_exit_to_user_mode+0x26/0x140 kernel/entry/common.c:297
>  do_syscall_64+0x4d/0xc0 arch/x86/entry/common.c:86
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> read to 0xffff888108bf8310 of 4 bytes by task 20448 on cpu 1:
>  io_timeout+0x88/0x270 io_uring/timeout.c:546
>  io_issue_sqe+0x147/0x660 io_uring/io_uring.c:1907
>  io_queue_sqe io_uring/io_uring.c:2079 [inline]
>  io_submit_sqe io_uring/io_uring.c:2340 [inline]
>  io_submit_sqes+0x689/0xfe0 io_uring/io_uring.c:2450
>  __do_sys_io_uring_enter io_uring/io_uring.c:3458 [inline]
>  __se_sys_io_uring_enter+0x1e5/0x1b70 io_uring/io_uring.c:3392
>  __x64_sys_io_uring_enter+0x78/0x90 io_uring/io_uring.c:3392
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> value changed: 0x00000c75 -> 0x00000c76
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 20448 Comm: syz-executor.2 Not tainted 6.3.0-rc4-syzkaller-00025-g3a93e40326c8 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/02/2023
> ==================================================================
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
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>
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