linux-kernel - Re: kernel BUG at fs/notify/dnotify/dnotify.c:LINE! (2)

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Date:   Wed, 9 Dec 2020 14:38:42 +0100
From:   Jan Kara <jack@...e.cz>
To:     syzbot <syzbot+f427adf9324b92652ccc@...kaller.appspotmail.com>
Cc:     amir73il@...il.com, jack@...e.cz, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: kernel BUG at fs/notify/dnotify/dnotify.c:LINE! (2)

Hello!

so I was debugging the dnotify crash below (it's 100% reproducible for me)
and I came to the following. The reproducer opens 'file0' on FUSE
filesystem which is a directory at that point. Then it attached dnotify
mark to the directory 'file0' and then it does something to the FUSE fs
which I don't understand but the result is that when FUSE is unmounted the
'file0' inode is actually a regular file (note that I've verified this is
really the same inode pointer). This then confuses dnotify which doesn't
tear down its structures properly and eventually crashes. So my question
is: How can an inode on FUSE filesystem morph from a dir to a regular file?
I presume this could confuse much more things than just dnotify?

Before I dwelve more into FUSE internals, any idea Miklos what could have
gone wrong and how to debug this further?

								Honza

On Mon 23-11-20 02:05:16, syzbot wrote:
> syzbot found the following issue on:
> 
> HEAD commit:    27bba9c5 Merge tag 'scsi-fixes' of git://git.kernel.org/pu..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=11b82225500000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=330f3436df12fd44
> dashboard link: https://syzkaller.appspot.com/bug?extid=f427adf9324b92652ccc
> compiler:       gcc (GCC) 10.1.0-syz 20200507
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11d3f015500000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17162d4d500000
> 
> Bisection is inconclusive: the issue happens on the oldest tested release.
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=16570525500000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=15570525500000
> console output: https://syzkaller.appspot.com/x/log.txt?x=11570525500000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+f427adf9324b92652ccc@...kaller.appspotmail.com
> 
> wlan1: Creating new IBSS network, BSSID 50:50:50:50:50:50
> ------------[ cut here ]------------
> kernel BUG at fs/notify/dnotify/dnotify.c:118!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 648 Comm: kworker/u4:4 Not tainted 5.10.0-rc4-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> Workqueue: events_unbound fsnotify_mark_destroy_workfn
> RIP: 0010:dnotify_free_mark fs/notify/dnotify/dnotify.c:118 [inline]
> RIP: 0010:dnotify_free_mark+0x4b/0x60 fs/notify/dnotify/dnotify.c:112
> Code: 80 3c 02 00 75 26 48 83 bd 80 00 00 00 00 75 15 e8 0a d3 a0 ff 48 89 ee 48 8b 3d 68 8c 1d 0b 5d e9 aa 06 e2 ff e8 f5 d2 a0 ff <0f> 0b e8 ae 4d e2 ff eb d3 66 90 66 2e 0f 1f 84 00 00 00 00 00 41
> RSP: 0018:ffffc90002f1fc38 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: ffffffff8958ae60 RCX: 1ffff920005e3f95
> RDX: ffff888012601a40 RSI: ffffffff81cf5ceb RDI: ffff88801aea2080
> RBP: ffff88801aea2000 R08: 0000000000000001 R09: ffffffff8ebb170f
> R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880171a2000
> R13: ffffc90002f1fc98 R14: ffff88801aea2010 R15: ffff88801aea2018
> FS:  0000000000000000(0000) GS:ffff8880b9f00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000056045fa95978 CR3: 0000000012121000 CR4: 00000000001506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  fsnotify_final_mark_destroy+0x71/0xb0 fs/notify/mark.c:205
>  fsnotify_mark_destroy_workfn+0x1eb/0x340 fs/notify/mark.c:840
>  process_one_work+0x933/0x15a0 kernel/workqueue.c:2272
>  worker_thread+0x64c/0x1120 kernel/workqueue.c:2418
>  kthread+0x3af/0x4a0 kernel/kthread.c:292
>  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296
> Modules linked in:
> 
> 
> ---
> 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.
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> syzbot can test patches for this issue, for details see:
> https://goo.gl/tpsmEJ#testing-patches
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR