linux-kernel - Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq

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Date: Thu, 9 May 2024 17:52:21 +0300
From: Amir Goldstein <amir73il@...il.com>
To: Hillf Danton <hdanton@...a.com>
Cc: syzbot <syzbot+4c493dcd5a68168a94b2@...kaller.appspotmail.com>, 
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>, 
	linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Pavel Machek <pavel@....cz>, linux-pm@...r.kernel.org
Subject: Re: [syzbot] [kernfs?] possible deadlock in kernfs_seq_start

On Thu, May 9, 2024 at 1:49 PM Hillf Danton <hdanton@...a.com> wrote:
>
> On Thu, 9 May 2024 09:37:24 +0300 Amir Goldstein <amir73il@...il.com>
> > On Thu, May 9, 2024 at 2:19 AM Hillf Danton <hdanton@...a.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://git.kern..
> > > > git tree:       upstream
> > > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=137daa6c980000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=9d7ea7de0cb32587
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=4c493dcd5a68168a94b2
> > > > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1134f3c0980000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1367a504980000
> > > >
> > > > Downloadable assets:
> > > > disk image: https://storage.googleapis.com/syzbot-assets/ea1961ce01fe/disk-dccb07f2.raw.xz
> > > > vmlinux: https://storage.googleapis.com/syzbot-assets/445a00347402/vmlinux-dccb07f2.xz
> > > > kernel image: https://storage.googleapis.com/syzbot-assets/461aed7c4df3/bzImage-dccb07f2.xz
> > > >
> > > > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > > > Reported-by: syzbot+4c493dcd5a68168a94b2@...kaller.appspotmail.com
> > > >
> > > > ======================================================
> > > > 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+0x53/0x3b0 fs/kernfs/file.c:154
> > > >
> > > > but task is already holding lock:
> > > > ffff88802d80b540 (&p->lock){+.+.}-{3:3}, at: seq_read_iter+0xb7/0xd60 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 = 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.