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From: Amir Goldstein <amir73il@gmail.com>
Date: Thu, 9 May 2024 17:52:21 +0300
Message-ID: <CAOQ4uxg8karas=5JxmCg0P5Wxhfzn41evgs_OUxd1GxBRpb4zQ@mail.gmail.com>
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq_start
To: Hillf Danton <hdanton@sina.com>
Cc: syzbot <syzbot+4c493dcd5a68168a94b2@syzkaller.appspotmail.com>, 
	linux-fsdevel@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>, 
	linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, 
	"Rafael J. Wysocki" <rafael@kernel.org>, Pavel Machek <pavel@ucw.cz>, linux-pm@vger.kernel.org
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On Thu, May 9, 2024 at 1:49=E2=80=AFPM Hillf Danton <hdanton@sina.com> wrot=
e:
>
> On Thu, 9 May 2024 09:37:24 +0300 Amir Goldstein <amir73il@gmail.com>
> > On Thu, May 9, 2024 at 2:19=E2=80=AFAM Hillf Danton <hdanton@sina.com> =
wrote:
> > > On Tue, 07 May 2024 22:36:18 -0700
> > > > syzbot has found a reproducer for the following issue on:
> > > >
> > > > HEAD commit:    dccb07f2914c Merge tag 'for-6.9-rc7-tag' of git://g=
it.kern..
> > > > git tree:       upstream
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=3D137daa6=
c980000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=3D9d7ea7d=
e0cb32587
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=3D4c493dcd5=
a68168a94b2
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils f=
or Debian) 2.40
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=3D1134f=
3c0980000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=3D1367a50=
4980000
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/ea1961ce01=
fe/disk-dccb07f2.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/445a00347402/=
vmlinux-dccb07f2.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/461aed7c=
4df3/bzImage-dccb07f2.xz
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to th=
e commit:
> > > > Reported-by: syzbot+4c493dcd5a68168a94b2@syzkaller.appspotmail.com
> > > >
> > > > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D
> > > > WARNING: possible circular locking dependency detected
> > > > 6.9.0-rc7-syzkaller-00012-gdccb07f2914c #0 Not tainted
> > > > ------------------------------------------------------
> > > > syz-executor149/5078 is trying to acquire lock:
> > > > ffff88802a978888 (&of->mutex){+.+.}-{3:3}, at: kernfs_seq_start+0x5=
3/0x3b0 fs/kernfs/file.c:154
> > > >
> > > > but task is already holding lock:
> > > > ffff88802d80b540 (&p->lock){+.+.}-{3:3}, at: seq_read_iter+0xb7/0xd=
60 fs/seq_file.c:182
> > > >
> > > > which lock already depends on the new lock.
> > > >
> > > >
> > > > the existing dependency chain (in reverse order) is:
> > > >
> > > > -> #4 (&p->lock){+.+.}-{3:3}:
> > > >        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
> > > >        __mutex_lock_common kernel/locking/mutex.c:608 [inline]
> > > >        __mutex_lock+0x136/0xd70 kernel/locking/mutex.c:752
> > > >        seq_read_iter+0xb7/0xd60 fs/seq_file.c:182
> > > >        call_read_iter include/linux/fs.h:2104 [inline]
> > > >        copy_splice_read+0x662/0xb60 fs/splice.c:365
> > > >        do_splice_read fs/splice.c:985 [inline]
> > > >        splice_file_to_pipe+0x299/0x500 fs/splice.c:1295
> > > >        do_sendfile+0x515/0xdc0 fs/read_write.c:1301
> > > >        __do_sys_sendfile64 fs/read_write.c:1362 [inline]
> > > >        __se_sys_sendfile64+0x17c/0x1e0 fs/read_write.c:1348
> > > >        do_syscall_x64 arch/x86/entry/common.c:52 [inline]
> > > >        do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
> > > >        entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > > >
> > > > -> #3 (&pipe->mutex){+.+.}-{3:3}:
> > > >        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
> > > >        __mutex_lock_common kernel/locking/mutex.c:608 [inline]
> > > >        __mutex_lock+0x136/0xd70 kernel/locking/mutex.c:752
> > > >        iter_file_splice_write+0x335/0x14e0 fs/splice.c:687
> > > >        backing_file_splice_write+0x2bc/0x4c0 fs/backing-file.c:289
> > > >        ovl_splice_write+0x3cf/0x500 fs/overlayfs/file.c:379
> > > >        do_splice_from fs/splice.c:941 [inline]
> > > >        do_splice+0xd77/0x1880 fs/splice.c:1354
>
>                 file_start_write(out);
>                 ret =3D do_splice_from(ipipe, out, &offset, len, flags);
>                 file_end_write(out);
>
> The correct locking order is
>
>                 sb_writers

This is sb of overlayfs

>                 inode lock

This is real inode

See comment above ovl_lockdep_annotate_inode_mutex_key()
for more details.

Thanks,
Amir.