Message-ID: <CANpmjNO7uiz=zKmTkXGO5cn1BUDuPNC3MS7Vdgua0i2bdqeMjQ@mail.gmail.com>
Date: Wed, 19 Jun 2024 20:49:37 +0200
From: Marco Elver <elver@...gle.com>
To: Santosh Pradhan <santosh.pradhan@...il.com>
Cc: syzbot <syzbot+c147f9175ec6cc7bd73b@...kaller.appspotmail.com>, axboe@...nel.dk, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [block?] KCSAN: data-race in block_uevent / inc_diskseq (2)

On Wed, 19 Jun 2024 at 19:00, Santosh Pradhan <santosh.pradhan@...il.com> wrote:
>
> Hi All,
> I guess this can be fixed by READ_ONCE(disk->diskseq) while reading
> and WRITE_ONCE(disk->diskseq, atomic64_inc_return(&diskseq)) in
> inc_diskseq() to avoid data race.
>
> I can probably send a patch if my understanding is correct.

What you propose fixes the underlying data race. However, I wonder if
there is a race condition that needs to be taken care of: whatever
sees the string "DISKSEQ=<seqnum>" can end up with either the old
value or new value (in this case 45 or 48 respectively). Is whoever
uses this string tolerant to the value randomly changing?

> Best Regards,
> Santosh
>
>
> On Wed, Jun 19, 2024 at 5:57 PM syzbot
> <syzbot+c147f9175ec6cc7bd73b@...kaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    3d54351c64e8 Merge tag 'lsm-pr-20240617' of git://git.kern..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=12426cfa980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=cd45aedbb3f7637b
> > dashboard link: https://syzkaller.appspot.com/bug?extid=c147f9175ec6cc7bd73b
> > 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/ba0a777cccff/disk-3d54351c.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/a78193318c92/vmlinux-3d54351c.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/54456f3d3bfe/bzImage-3d54351c.xz
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+c147f9175ec6cc7bd73b@...kaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KCSAN: data-race in block_uevent / inc_diskseq
> >
> > write to 0xffff888101f05220 of 8 bytes by task 3101 on cpu 0:
> >  inc_diskseq+0x2c/0x40 block/genhd.c:1472
> >  disk_force_media_change+0x9f/0xf0 block/disk-events.c:297
> >  __loop_clr_fd+0x270/0x3f0 drivers/block/loop.c:1193
> >  loop_clr_fd drivers/block/loop.c:1276 [inline]
> >  lo_ioctl+0xea6/0x1330 drivers/block/loop.c:1578
> >  blkdev_ioctl+0x35f/0x450 block/ioctl.c:676
> >  vfs_ioctl fs/ioctl.c:51 [inline]
> >  __do_sys_ioctl fs/ioctl.c:907 [inline]
> >  __se_sys_ioctl+0xd3/0x150 fs/ioctl.c:893
> >  __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:893
> >  x64_sys_call+0x1581/0x2d70 arch/x86/include/generated/asm/syscalls_64.h:17
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > read to 0xffff888101f05220 of 8 bytes by task 3091 on cpu 1:
> >  block_uevent+0x31/0x50 block/genhd.c:1206
> >  dev_uevent+0x2f3/0x380 drivers/base/core.c:2687
> >  uevent_show+0x11e/0x210 drivers/base/core.c:2745
> >  dev_attr_show+0x3a/0xa0 drivers/base/core.c:2437
> >  sysfs_kf_seq_show+0x17c/0x250 fs/sysfs/file.c:59
> >  kernfs_seq_show+0x7c/0x90 fs/kernfs/file.c:205
> >  seq_read_iter+0x2d7/0x940 fs/seq_file.c:230
> >  kernfs_fop_read_iter+0xc6/0x310 fs/kernfs/file.c:279
> >  new_sync_read fs/read_write.c:395 [inline]
> >  vfs_read+0x5e6/0x6e0 fs/read_write.c:476
> >  ksys_read+0xeb/0x1b0 fs/read_write.c:619
> >  __do_sys_read fs/read_write.c:629 [inline]
> >  __se_sys_read fs/read_write.c:627 [inline]
> >  __x64_sys_read+0x42/0x50 fs/read_write.c:627
> >  x64_sys_call+0x27e5/0x2d70 arch/x86/include/generated/asm/syscalls_64.h:1
> >  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> >  do_syscall_64+0xc9/0x1c0 arch/x86/entry/common.c:83
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> >
> > value changed: 0x0000000000000045 -> 0x0000000000000048
> >
> > Reported by Kernel Concurrency Sanitizer on:
> > CPU: 1 PID: 3091 Comm: udevd Not tainted 6.10.0-rc4-syzkaller-00035-g3d54351c64e8 #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:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
> >
> > If the report is already addressed, let syzbot know by replying with:
> > #syz fix: exact-commit-title
> >
> > If you want to overwrite report's subsystems, reply with:
> > #syz set subsystems: new-subsystem
> > (See the list of subsystem names on the web dashboard)
> >
> > If the report is a duplicate of another one, reply with:
> > #syz dup: exact-subject-of-another-report
> >
> > If you want to undo deduplication, reply with:
> > #syz undup
> >
>
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