[syzbot] [block?] KCSAN: data-race in block_uevent / inc_diskseq (2)

[syzbot]

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+c147f9...@syzkaller.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
==================================================================


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[Santosh Pradhan]

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.

Best Regards,
Santosh

[Marco Elver]

On Wed, 19 Jun 2024 at 19:00, Santosh Pradhan <santosh...@gmail.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?

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