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Subject: Re: [syzbot] [reiserfs?] possible deadlock in open_xa_dir
From:   Roberto Sassu <roberto.sassu@huaweicloud.com>
To:     Jeff Mahoney <jeffm@suse.com>, Paul Moore <paul@paul-moore.com>,
        syzbot <syzbot+8fb64a61fdd96b50f3b8@syzkaller.appspotmail.com>
Cc:     hdanton@sina.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, reiserfs-devel@vger.kernel.org,
        roberto.sassu@huawei.com, syzkaller-bugs@googlegroups.com,
        peterz@infradead.org, mingo@redhat.com, will@kernel.org,
        Jan Kara <jack@suse.cz>
Date:   Fri, 02 Jun 2023 09:20:06 +0200
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On Thu, 2023-06-01 at 17:22 -0400, Jeff Mahoney wrote:
> On 5/31/23 05:49, Roberto Sassu wrote:
> > On 5/5/2023 11:36 PM, Paul Moore wrote:
> > > On Fri, May 5, 2023 at 4:51 PM syzbot
> > > <syzbot+8fb64a61fdd96b50f3b8@syzkaller.appspotmail.com> wrote:
> > > > syzbot has bisected this issue to:
> > > > 
> > > > commit d82dcd9e21b77d338dc4875f3d4111f0db314a7c
> > > > Author: Roberto Sassu <roberto.sassu@huawei.com>
> > > > Date:   Fri Mar 31 12:32:18 2023 +0000
> > > > 
> > > >      reiserfs: Add security prefix to xattr name in 
> > > > reiserfs_security_write()
> > > > 
> > > > bisection log:  
> > > > https://syzkaller.appspot.com/x/bisect.txt?x=14403182280000
> > > > start commit:   3c4aa4434377 Merge tag 'ceph-for-6.4-rc1' of 
> > > > https://githu..
> > > > git tree:       upstream
> > > > final oops:     
> > > > https://syzkaller.appspot.com/x/report.txt?x=16403182280000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=12403182280000
> > > > kernel config:  
> > > > https://syzkaller.appspot.com/x/.config?x=73a06f6ef2d5b492
> > > > dashboard link: 
> > > > https://syzkaller.appspot.com/bug?extid=8fb64a61fdd96b50f3b8
> > > > syz repro:      
> > > > https://syzkaller.appspot.com/x/repro.syz?x=12442414280000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=176a7318280000
> > > > 
> > > > Reported-by: syzbot+8fb64a61fdd96b50f3b8@syzkaller.appspotmail.com
> > > > Fixes: d82dcd9e21b7 ("reiserfs: Add security prefix to xattr name in 
> > > > reiserfs_security_write()")
> > > > 
> > > > For information about bisection process see: 
> > > > https://goo.gl/tpsmEJ#bisection
> > > 
> > > I don't think Roberto's patch identified above is the actual root
> > > cause of this problem as reiserfs_xattr_set_handle() is called in
> > > reiserfs_security_write() both before and after the patch.  However,
> > > due to some bad logic in reiserfs_security_write() which Roberto
> > > corrected, I'm thinking that it is possible this code is being
> > > exercised for the first time and syzbot is starting to trigger a
> > > locking issue in the reiserfs code ... ?
> > 
> > + Jan, Jeff (which basically restructured the lock)
> > 
> > + Petr, Ingo, Will

Peter, clearly (sorry!)

> I involve the lockdep experts, to get a bit of help on this.
> 
> Yep, looks like that's been broken since it was added in 2009.  Since 
> there can't be any users of it, it'd make sense to drop the security 
> xattr support from reiserfs entirely.

Thanks, Jeff. Will make a patch to implement your suggestion.

Meanwhile, I learned how to read lockdep a bit better. The following
format could have helped me to understand it more quickly. The proposal
is simply to change #n to CPU#n at the top of the trace, define labels
L#n for the locks, and add them where effectively are held.

[   77.746561][ T5418] -> CPU1 (&sbi->lock){+.+.}-{3:3}:
[   77.753772][ T5418]        lock_acquire+0x23e/0x630
[   77.758792][ T5418]        __mutex_lock_common+0x1d8/0x2530
[   77.764504][ T5418]   (L3) mutex_lock_nested+0x1b/0x20
[   77.769868][ T5418]        reiserfs_write_lock+0x70/0xc0
[   77.775321][ T5418]        reiserfs_mkdir+0x321/0x870
[   77.780509][ T5418]   (L2) open_xa_dir+0x259/0x540
[   77.785440][ T5418]        xattr_lookup+0x17/0x210
[   77.790378][ T5418]        reiserfs_xattr_set_handle+0xda/0xc80
[   77.796448][ T5418]        reiserfs_security_write+0x134/0x190
[   77.802416][ T5418]        reiserfs_new_inode+0x13bf/0x1a90
[   77.808124][ T5418]        reiserfs_create+0x3b1/0x680
[   77.813399][ T5418]        path_openat+0xf1e/0x2c10
[   77.818415][ T5418]        do_filp_open+0x22a/0x440
[   77.823433][ T5418]        do_sys_openat2+0x10f/0x430
[   77.828624][ T5418]        __x64_sys_creat+0x11e/0x160
[   77.833905][ T5418]        do_syscall_64+0x41/0xc0
[   77.838926][ T5418]        entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   77.845514][ T5418] 
[   77.845514][ T5418] -> CPU0 (&type->i_mutex_dir_key#8/3){+.+.}-{3:3}:
[   77.854118][ T5418]        validate_chain+0x166b/0x58e0
[   77.859488][ T5418]        __lock_acquire+0x125b/0x1f80
[   77.864853][ T5418]        lock_acquire+0x23e/0x630
[   77.869909][ T5418]   (L4) down_write_nested+0x3d/0x50
[   77.875186][ T5418]        open_xa_dir+0x134/0x540
[   77.880117][ T5418]        xattr_lookup+0x17/0x210
[   77.885050][ T5418]        reiserfs_xattr_get+0xe1/0x4a0
[   77.890501][ T5418]        __vfs_getxattr+0x2fe/0x350
[   77.895802][ T5418]        smk_fetch+0x98/0xf0
[   77.900382][ T5418]        smack_d_instantiate+0x5d5/0xa20
[   77.906018][ T5418]        security_d_instantiate+0x6b/0xb0
[   77.911736][ T5418]        d_instantiate_new+0x5e/0xe0
[   77.917013][ T5418]   (L1) reiserfs_create+0x5ee/0x680
[   77.922293][ T5418]        path_openat+0xf1e/0x2c10
[   77.927308][ T5418]        do_filp_open+0x22a/0x440
[   77.932330][ T5418]        do_sys_openat2+0x10f/0x430
[   77.937515][ T5418]        __x64_sys_creat+0x11e/0x160
[   77.942874][ T5418]        do_syscall_64+0x41/0xc0
[   77.947796][ T5418]        entry_SYSCALL_64_after_hwframe+0x63/0xcd
[   77.954200][ T5418] 
[   77.954200][ T5418] other info that might help us debug this:
[   77.954200][ T5418] 
[   77.964508][ T5418]  Possible unsafe locking scenario:
[   77.964508][ T5418] 
[   77.972034][ T5418]   CPU0                         CPU1
[   77.977394][ T5418]   ----                         ----
[   77.982748][ T5418]   L1: lock(&sbi->lock);
[   77.986726][ T5418]                                L2: lock(&type->i_mutex_dir_key#8/3);
[   77.994618][ T5418]                                L3: lock(&sbi->lock);
[   78.001118][ T5418]   L4: lock(&type->i_mutex_dir_key#8/3);

Thanks

Roberto

> > First of all, the lockdep warning is trivial to reproduce:
> > 
> > # dd if=/dev/zero of=reiserfs.img bs=1M count=100
> > # losetup -f --show reiserfs.img
> > /dev/loop0
> > # mkfs.reiserfs /dev/loop0
> > # mount /dev/loop0 /mnt/
> > # touch file0
> > 
> > In the testing system, Smack is the major LSM.
> > 
> > Ok, so the warning here is clear:
> > 
> > https://syzkaller.appspot.com/x/log.txt?x=12403182280000
> > 
> > However, I was looking if that can really happen. From this:
> > 
> > [   77.746561][ T5418] -> #1 (&sbi->lock){+.+.}-{3:3}:
> > [   77.753772][ T5418]        lock_acquire+0x23e/0x630
> > [   77.758792][ T5418]        __mutex_lock_common+0x1d8/0x2530
> > [   77.764504][ T5418]        mutex_lock_nested+0x1b/0x20
> > [   77.769868][ T5418]        reiserfs_write_lock+0x70/0xc0
> > [   77.775321][ T5418]        reiserfs_mkdir+0x321/0x870
> > 
> > I see that the lock is taken in reiserfs_write_lock(), while lockdep says:
> > 
> > [   77.710227][ T5418] but task is already holding lock:
> > [   77.717587][ T5418] ffff88807568d090 (&sbi->lock){+.+.}-{3:3}, at: 
> > reiserfs_write_lock_nested+0x4a/0xb0
> > 
> > which is in a different place, I believe here:
> > 
> > int reiserfs_paste_into_item(struct reiserfs_transaction_handle *th,
> >                               /* Path to the pasted item. */
> > [...]
> > 
> >          depth = reiserfs_write_unlock_nested(sb);
> >          dquot_free_space_nodirty(inode, pasted_size);
> >          reiserfs_write_lock_nested(sb, depth);
> >          return retval;
> > }
> > 
> > This is called by reiserfs_add_entry(), which is called by 
> > reiserfs_create() (it is in the lockdep trace). After returning to 
> > reiserfs_create(), d_instantiate_new() is called.
> > 
> > I don't know exactly, I take the part that the lock is held. But if it 
> > is held, how d_instantiate_new() can be executed in another task?
> > 
> > static int reiserfs_create(struct mnt_idmap *idmap, struct inode *dir,
> >                          struct dentry *dentry, umode_t mode, bool excl)
> > {
> > 
> > [...]
> > 
> >          reiserfs_write_lock(dir->i_sb);
> > 
> >          retval = journal_begin(&th, dir->i_sb, jbegin_count);
> > 
> > [...]
> > 
> >          d_instantiate_new(dentry, inode);
> >          retval = journal_end(&th);
> > 
> > out_failed:
> >          reiserfs_write_unlock(dir->i_sb);
> > 
> > If the lock is held, the scenario lockdep describes cannot happen. Any 
> > thoughts?
> 
> It's important to understand that the reiserfs write lock was added as a 
> subsystem-specific replacement for the BKL.  Given that reiserfs was 
> dying already back then, it made more sense from a time management 
> perspective to emulate that behavior internally rather than use new 
> locking when practically nobody cared anymore.
> 
> See reiserfs_write_unlock_nested and reiserfs_write_lock_nested paired 
> throughout the code.  It drops the lock when it passes a point where 
> it's likely to schedule, just like the BKL would have.
> 
> Yes, it's a mess.  Just let it die quietly.
> 
> -Jeff
>