Re: [syzbot] [btrfs?] KASAN: slab-use-after-free Read in btrfs_open_devices

[Christoph Hellwig <hch@lst.de>]

Yes, probably.  The lifetimes looked fishy to me to start with, but
this might have made things worse.

On Tue, Aug 08, 2023 at 05:50:02PM +0200, Christian Brauner wrote:
> On Mon, Aug 07, 2023 at 08:24:36PM -0700, syzbot wrote:
> > syzbot has bisected this issue to:
> > 
> > commit 066d64b26a21a5b5c500a30f27f3e4b1959aac9e
> > Author: Christoph Hellwig <hch@lst.de>
> > Date:   Wed Aug 2 15:41:23 2023 +0000
> > 
> >     btrfs: open block devices after superblock creation
> > 
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15493371a80000
> > start commit:   f7dc24b34138 Add linux-next specific files for 20230807
> > git tree:       linux-next
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=17493371a80000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13493371a80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=d7847c9dca13d6c5
> > dashboard link: https://syzkaller.appspot.com/bug?extid=26860029a4d562566231
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=179704c9a80000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17868ba9a80000
> > 
> > Reported-by: syzbot+26860029a4d562566231@syzkaller.appspotmail.com
> > Fixes: 066d64b26a21 ("btrfs: open block devices after superblock creation")
> > 
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> 
> I think the issue might be that before your patch the lifetime of:
> @device was aligned with @device->s_fs_info but now that you're dropping
> the uuid mutex after btrfs_scan_one_device() that isn't true anymore. So
> it feels like:
> 
> P1                                       P2
> lock_uuid_mutex;
> device = btrfs_scan_one_device();
> fs_devices = device->fs_devices;
> unlock_uuid_mutex;
>                                          // earlier mount that gets cleaned up
>                                          lock_uuid_mutex; 
> 					 btrfs_close_devices(fs_devices);
>                                          unlock_uuid_mutex;
> 
> lock_uuid_mutex;
> btrfs_open_devices(fs_devices); // UAF
> unlock_uuid_mutex;
> 
> But I'm not entirely sure.
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