Re: [syzbot] [reiserfs?] possible deadlock in open_xa_dir
From: Roberto Sassu
Date: Fri Jun 02 2023 - 03:18:30 EST
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@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
> > > > syzbot has bisected this issue to:
> > > >
> > > > commit d82dcd9e21b77d338dc4875f3d4111f0db314a7c
> > > > Author: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > > > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > > 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
>